diff --git a/Bio/Align/_pairwisealigner.c b/Bio/Align/_pairwisealigner.c
index 499bbdf59..f1805b095 100644
--- a/Bio/Align/_pairwisealigner.c
+++ b/Bio/Align/_pairwisealigner.c
@@ -1961,11 +1961,12 @@ Aligner_str(Aligner* self)
 {
     char text[1024];
     char* p = text;
+    char* value;
     PyObject* substitution_matrix = self->substitution_matrix.obj;
     void* args[3];
     int n = 0;
     PyObject* wildcard = NULL;
-    PyObject* s;
+    PyObject* s = NULL;
 
     p += sprintf(p, "Pairwise sequence aligner with parameters\n");
     if (substitution_matrix) {
@@ -1983,44 +1984,88 @@ Aligner_str(Aligner* self)
             p += sprintf(p, "  wildcard: '%%U'\n");
             args[n++] = wildcard;
         }
-        p += sprintf(p, "  match_score: %f\n", self->match);
-        p += sprintf(p, "  mismatch_score: %f\n", self->mismatch);
+        /* Use PyOS_double_to_string to ensure that the locale does
+         * not change the decimal point into a comma.
+         */
+        value = PyOS_double_to_string(self->match, 'f', 6, 0, NULL);
+        if (!value) goto exit;
+        p += sprintf(p, "  match_score: %s\n", value);
+        PyMem_Free(value);
+        value = PyOS_double_to_string(self->mismatch, 'f', 6, 0, NULL);
+        if (!value) goto exit;
+        p += sprintf(p, "  mismatch_score: %s\n", value);
+        PyMem_Free(value);
     }
     if (self->insertion_score_function) {
         p += sprintf(p, "  insertion_score_function: %%R\n");
         args[n++] = self->insertion_score_function;
     }
     else {
-        p += sprintf(p, "  open_internal_insertion_score: %f\n",
-                     self->open_internal_insertion_score);
-        p += sprintf(p, "  extend_internal_insertion_score: %f\n",
-                     self->extend_internal_insertion_score);
-        p += sprintf(p, "  open_left_insertion_score: %f\n",
-                     self->open_left_insertion_score);
-        p += sprintf(p, "  extend_left_insertion_score: %f\n",
-                     self->extend_left_insertion_score);
-        p += sprintf(p, "  open_right_insertion_score: %f\n",
-                     self->open_right_insertion_score);
-        p += sprintf(p, "  extend_right_insertion_score: %f\n",
-                     self->extend_right_insertion_score);
+        value = PyOS_double_to_string(self->open_internal_insertion_score,
+                                      'f', 6, 0, NULL);
+        if (!value) goto exit;
+        p += sprintf(p, "  open_internal_insertion_score: %s\n", value);
+        PyMem_Free(value);
+        value = PyOS_double_to_string(self->extend_internal_insertion_score,
+                                      'f', 6, 0, NULL);
+        if (!value) goto exit;
+        p += sprintf(p, "  extend_internal_insertion_score: %s\n", value);
+        PyMem_Free(value);
+        value = PyOS_double_to_string(self->open_left_insertion_score,
+                                      'f', 6, 0, NULL);
+        if (!value) goto exit;
+        p += sprintf(p, "  open_left_insertion_score: %s\n", value);
+        PyMem_Free(value);
+        value = PyOS_double_to_string(self->extend_left_insertion_score,
+                                      'f', 6, 0, NULL);
+        if (!value) goto exit;
+        p += sprintf(p, "  extend_left_insertion_score: %s\n", value);
+        PyMem_Free(value);
+        value = PyOS_double_to_string(self->open_right_insertion_score,
+                                      'f', 6, 0, NULL);
+        if (!value) goto exit;
+        p += sprintf(p, "  open_right_insertion_score: %s\n", value);
+        PyMem_Free(value);
+        value = PyOS_double_to_string(self->extend_right_insertion_score,
+                                      'f', 6, 0, NULL);
+        if (!value) goto exit;
+        p += sprintf(p, "  extend_right_insertion_score: %s\n", value);
+        PyMem_Free(value);
     }
     if (self->deletion_score_function) {
         p += sprintf(p, "  deletion_score_function: %%R\n");
         args[n++] = self->deletion_score_function;
     }
     else {
-        p += sprintf(p, "  open_internal_deletion_score: %f\n",
-                     self->open_internal_deletion_score);
-        p += sprintf(p, "  extend_internal_deletion_score: %f\n",
-                     self->extend_internal_deletion_score);
-        p += sprintf(p, "  open_left_deletion_score: %f\n",
-                     self->open_left_deletion_score);
-        p += sprintf(p, "  extend_left_deletion_score: %f\n",
-                     self->extend_left_deletion_score);
-        p += sprintf(p, "  open_right_deletion_score: %f\n",
-                     self->open_right_deletion_score);
-        p += sprintf(p, "  extend_right_deletion_score: %f\n",
-                     self->extend_right_deletion_score);
+        value = PyOS_double_to_string(self->open_internal_deletion_score,
+                                      'f', 6, 0, NULL);
+        if (!value) goto exit;
+        p += sprintf(p, "  open_internal_deletion_score: %s\n", value);
+        PyMem_Free(value);
+        value = PyOS_double_to_string(self->extend_internal_deletion_score,
+                                      'f', 6, 0, NULL);
+        p += sprintf(p, "  extend_internal_deletion_score: %s\n", value);
+        PyMem_Free(value);
+        value = PyOS_double_to_string(self->open_left_deletion_score,
+                                      'f', 6, 0, NULL);
+        if (!value) goto exit;
+        p += sprintf(p, "  open_left_deletion_score: %s\n", value);
+        PyMem_Free(value);
+        value = PyOS_double_to_string(self->extend_left_deletion_score,
+                                      'f', 6, 0, NULL);
+        if (!value) goto exit;
+        p += sprintf(p, "  extend_left_deletion_score: %s\n", value);
+        PyMem_Free(value);
+        value = PyOS_double_to_string(self->open_right_deletion_score,
+                                      'f', 6, 0, NULL);
+        if (!value) goto exit;
+        p += sprintf(p, "  open_right_deletion_score: %s\n", value);
+        PyMem_Free(value);
+        value = PyOS_double_to_string(self->extend_right_deletion_score,
+                                      'f', 6, 0, NULL);
+        if (!value) goto exit;
+        p += sprintf(p, "  extend_right_deletion_score: %s\n", value);
+        PyMem_Free(value);
     }
     switch (self->mode) {
         case Global: sprintf(p, "  mode: global\n"); break;
@@ -2031,6 +2076,8 @@ Aligner_str(Aligner* self)
             return NULL;
     }
     s = PyUnicode_FromFormat(text, args[0], args[1], args[2]);
+
+exit:
     Py_XDECREF(wildcard);
     return s;
 }
diff --git a/Bio/Cluster/clustermodule.c b/Bio/Cluster/clustermodule.c
index 87c2cbd14..a2005b319 100644
--- a/Bio/Cluster/clustermodule.c
+++ b/Bio/Cluster/clustermodule.c
@@ -682,8 +682,15 @@ PyNode_repr(PyNode* self)
 {
     char string[64];
 
-    sprintf(string, "(%d, %d): %g",
-                   self->node.left, self->node.right, self->node.distance);
+    /* Use PyOS_double_to_string to ensure that the locale does
+     * not change the decimal point into a comma.
+     */
+    char* value = PyOS_double_to_string(self->node.distance, 'g', 6, 0, NULL);
+    if (!value) return NULL;
+
+    sprintf(string, "(%d, %d): %s", self->node.left, self->node.right, value);
+    PyMem_Free(value);
+
     return PyUnicode_FromString(string);
 }
 
@@ -928,15 +935,27 @@ PyTree_str(PyTree* self)
     int i;
     const int n = self->n;
     char string[128];
+    char* distance;
     Node node;
     PyObject* line;
     PyObject* output;
     PyObject* temp;
 
     output = PyUnicode_FromString("");
+    if (!output) return NULL;
+
     for (i = 0; i < n; i++) {
         node = self->nodes[i];
-        sprintf(string, "(%d, %d): %g", node.left, node.right, node.distance);
+        distance = PyOS_double_to_string(node.distance, 'g', 6, 0, NULL);
+        if (!distance) {
+            Py_DECREF(output);
+            return NULL;
+        }
+        /* Use PyOS_double_to_string to ensure that the locale does
+         * not change the decimal point into a comma.
+         */
+        sprintf(string, "(%d, %d): %s", node.left, node.right, distance);
+        PyMem_Free(distance);
         if (i < n-1) strcat(string, "\n");
         line = PyUnicode_FromString(string);
         if (!line) {
@@ -944,11 +963,9 @@ PyTree_str(PyTree* self)
             return NULL;
         }
         temp = PyUnicode_Concat(output, line);
-        if (!temp) {
-            Py_DECREF(output);
-            Py_DECREF(line);
-            return NULL;
-        }
+        Py_DECREF(line);
+        Py_DECREF(output);
+        if (!temp) return NULL;
         output = temp;
     }
     return output;