diff --git a/CMakeLists.txt b/CMakeLists.txt
index 5a682bf7..47366c49 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -375,7 +375,7 @@ endif()
 if (MI_BUILD_TESTS)
   enable_testing()
 
-  foreach(TEST_NAME api stress)
+  foreach(TEST_NAME api api-fill stress)
     add_executable(mimalloc-test-${TEST_NAME} test/test-${TEST_NAME}.c)
     target_compile_definitions(mimalloc-test-${TEST_NAME} PRIVATE ${mi_defines})
     target_compile_options(mimalloc-test-${TEST_NAME} PRIVATE ${mi_cflags})
diff --git a/test/test-api-fill.c b/test/test-api-fill.c
new file mode 100644
index 00000000..404dcb7f
--- /dev/null
+++ b/test/test-api-fill.c
@@ -0,0 +1,180 @@
+/* ----------------------------------------------------------------------------
+Copyright (c) 2018-2020, Microsoft Research, Daan Leijen
+This is free software; you can redistribute it and/or modify it under the
+terms of the MIT license. A copy of the license can be found in the file
+"LICENSE" at the root of this distribution.
+-----------------------------------------------------------------------------*/
+#include "mimalloc.h"
+#include "mimalloc-types.h"
+
+#include "testhelper.h"
+
+// ---------------------------------------------------------------------------
+// Helper functions
+// ---------------------------------------------------------------------------
+bool check_zero_init(uint8_t* p, size_t size);
+
+// ---------------------------------------------------------------------------
+// Main testing
+// ---------------------------------------------------------------------------
+int main(void) {
+  mi_option_disable(mi_option_verbose);
+
+  // ---------------------------------------------------
+  // Zeroing allocation
+  // ---------------------------------------------------
+  CHECK_BODY("zeroinit-zalloc-small", {
+    size_t zalloc_size = MI_SMALL_SIZE_MAX / 2;
+    uint8_t* p = (uint8_t*)mi_zalloc(zalloc_size);
+    result = check_zero_init(p, zalloc_size);
+    mi_free(p);
+  });
+  CHECK_BODY("zeroinit-zalloc-large", {
+    size_t zalloc_size = MI_SMALL_SIZE_MAX * 2;
+    uint8_t* p = (uint8_t*)mi_zalloc(zalloc_size);
+    result = check_zero_init(p, zalloc_size);
+    mi_free(p);
+  });
+  CHECK_BODY("zeroinit-zalloc_small", {
+    size_t zalloc_size = MI_SMALL_SIZE_MAX / 2;
+    uint8_t* p = (uint8_t*)mi_zalloc_small(zalloc_size);
+    result = check_zero_init(p, zalloc_size);
+    mi_free(p);
+  });
+
+  CHECK_BODY("zeroinit-calloc-small", {
+    size_t calloc_size = MI_SMALL_SIZE_MAX / 2;
+    uint8_t* p = (uint8_t*)mi_calloc(calloc_size, 1);
+    result = check_zero_init(p, calloc_size);
+    mi_free(p);
+  });
+  CHECK_BODY("zeroinit-calloc-large", {
+    size_t calloc_size = MI_SMALL_SIZE_MAX * 2;
+    uint8_t* p = (uint8_t*)mi_calloc(calloc_size, 1);
+    result = check_zero_init(p, calloc_size);
+    mi_free(p);
+  });
+
+  CHECK_BODY("zeroinit-rezalloc-small", {
+    size_t zalloc_size = MI_SMALL_SIZE_MAX / 2;
+    uint8_t* p = (uint8_t*)mi_zalloc(zalloc_size);
+    result = check_zero_init(p, zalloc_size);
+    zalloc_size *= 3;
+    p = (uint8_t*)mi_rezalloc(p, zalloc_size);
+    result &= check_zero_init(p, zalloc_size);
+    mi_free(p);
+  });
+  CHECK_BODY("zeroinit-rezalloc-large", {
+    size_t zalloc_size = MI_SMALL_SIZE_MAX * 2;
+    uint8_t* p = (uint8_t*)mi_zalloc(zalloc_size);
+    result = check_zero_init(p, zalloc_size);
+    zalloc_size *= 3;
+    p = (uint8_t*)mi_rezalloc(p, zalloc_size);
+    result &= check_zero_init(p, zalloc_size);
+    mi_free(p);
+  });
+
+  CHECK_BODY("zeroinit-recalloc-small", {
+    size_t calloc_size = MI_SMALL_SIZE_MAX / 2;
+    uint8_t* p = (uint8_t*)mi_calloc(calloc_size, 1);
+    result = check_zero_init(p, calloc_size);
+    calloc_size *= 3;
+    p = (uint8_t*)mi_recalloc(p, calloc_size, 1);
+    result &= check_zero_init(p, calloc_size);
+    mi_free(p);
+  });
+  CHECK_BODY("zeroinit-recalloc-large", {
+    size_t calloc_size = MI_SMALL_SIZE_MAX * 2;
+    uint8_t* p = (uint8_t*)mi_calloc(calloc_size, 1);
+    result = check_zero_init(p, calloc_size);
+    calloc_size *= 3;
+    p = (uint8_t*)mi_recalloc(p, calloc_size, 1);
+    result &= check_zero_init(p, calloc_size);
+    mi_free(p);
+  });
+
+  // ---------------------------------------------------
+  // Zeroing in aligned API
+  // ---------------------------------------------------
+  CHECK_BODY("zeroinit-zalloc_aligned-small", {
+    size_t zalloc_size = MI_SMALL_SIZE_MAX / 2;
+    uint8_t* p = (uint8_t*)mi_zalloc_aligned(zalloc_size, MI_MAX_ALIGN_SIZE * 2);
+    result = check_zero_init(p, zalloc_size);
+    mi_free(p);
+  });
+  CHECK_BODY("zeroinit-zalloc_aligned-large", {
+    size_t zalloc_size = MI_SMALL_SIZE_MAX * 2;
+    uint8_t* p = (uint8_t*)mi_zalloc_aligned(zalloc_size, MI_MAX_ALIGN_SIZE * 2);
+    result = check_zero_init(p, zalloc_size);
+    mi_free(p);
+  });
+
+  CHECK_BODY("zeroinit-calloc_aligned-small", {
+    size_t calloc_size = MI_SMALL_SIZE_MAX / 2;
+    uint8_t* p = (uint8_t*)mi_calloc_aligned(calloc_size, 1, MI_MAX_ALIGN_SIZE * 2);
+    result = check_zero_init(p, calloc_size);
+    mi_free(p);
+  });
+  CHECK_BODY("zeroinit-calloc_aligned-large", {
+    size_t calloc_size = MI_SMALL_SIZE_MAX * 2;
+    uint8_t* p = (uint8_t*)mi_calloc_aligned(calloc_size, 1, MI_MAX_ALIGN_SIZE * 2);
+    result = check_zero_init(p, calloc_size);
+    mi_free(p);
+  });
+
+  CHECK_BODY("zeroinit-rezalloc_aligned-small", {
+    size_t zalloc_size = MI_SMALL_SIZE_MAX / 2;
+    uint8_t* p = (uint8_t*)mi_zalloc_aligned(zalloc_size, MI_MAX_ALIGN_SIZE * 2);
+    result = check_zero_init(p, zalloc_size);
+    zalloc_size *= 3;
+    p = (uint8_t*)mi_rezalloc_aligned(p, zalloc_size, MI_MAX_ALIGN_SIZE * 2);
+    result &= check_zero_init(p, zalloc_size);
+    mi_free(p);
+  });
+  CHECK_BODY("zeroinit-rezalloc_aligned-large", {
+    size_t zalloc_size = MI_SMALL_SIZE_MAX * 2;
+    uint8_t* p = (uint8_t*)mi_zalloc_aligned(zalloc_size, MI_MAX_ALIGN_SIZE * 2);
+    result = check_zero_init(p, zalloc_size);
+    zalloc_size *= 3;
+    p = (uint8_t*)mi_rezalloc_aligned(p, zalloc_size, MI_MAX_ALIGN_SIZE * 2);
+    result &= check_zero_init(p, zalloc_size);
+    mi_free(p);
+  });
+
+  CHECK_BODY("zeroinit-recalloc_aligned-small", {
+    size_t calloc_size = MI_SMALL_SIZE_MAX / 2;
+    uint8_t* p = (uint8_t*)mi_calloc_aligned(calloc_size, 1, MI_MAX_ALIGN_SIZE * 2);
+    result = check_zero_init(p, calloc_size);
+    calloc_size *= 3;
+    p = (uint8_t*)mi_recalloc_aligned(p, calloc_size, 1, MI_MAX_ALIGN_SIZE * 2);
+    result &= check_zero_init(p, calloc_size);
+    mi_free(p);
+  });
+  CHECK_BODY("zeroinit-recalloc_aligned-large", {
+    size_t calloc_size = MI_SMALL_SIZE_MAX * 2;
+    uint8_t* p = (uint8_t*)mi_calloc_aligned(calloc_size, 1, MI_MAX_ALIGN_SIZE * 2);
+    result = check_zero_init(p, calloc_size);
+    calloc_size *= 3;
+    p = (uint8_t*)mi_recalloc_aligned(p, calloc_size, 1, MI_MAX_ALIGN_SIZE * 2);
+    result &= check_zero_init(p, calloc_size);
+    mi_free(p);
+  });
+
+  // ---------------------------------------------------
+  // Done
+  // ---------------------------------------------------[]
+  return print_test_summary();
+}
+
+// ---------------------------------------------------------------------------
+// Helper functions
+// ---------------------------------------------------------------------------
+bool check_zero_init(uint8_t* p, size_t size) {
+  if(!p)
+    return false;
+  bool result = true;
+  for (size_t i = 0; i < size; ++i) {
+    result &= p[i] == 0;
+  }
+  return result;
+}