diff --git a/icu4c/source/i18n/tzdat.h b/icu4c/source/i18n/tzdat.h
index 31af094d79f..f75811391f4 100644
--- a/icu4c/source/i18n/tzdat.h
+++ b/icu4c/source/i18n/tzdat.h
@@ -63,7 +63,7 @@
 // Information used to identify and validate the data
 
 #define TZ_DATA_NAME "tz"
-#define TZ_DATA_TYPE "dat"
+#define TZ_DATA_TYPE "icu"
 
 // Fields in UDataInfo:
 
diff --git a/icu4c/source/i18n/ucol_imp.h b/icu4c/source/i18n/ucol_imp.h
index 2c13dcf29f7..4beb2954933 100644
--- a/icu4c/source/i18n/ucol_imp.h
+++ b/icu4c/source/i18n/ucol_imp.h
@@ -402,9 +402,9 @@ ucol_cloneRuleData(const UCollator *coll, int32_t *length, UErrorCode *status);
 #define getExpansionSuffix(coleiter) ((coleiter)->iteratordata_.CEpos - (coleiter)->iteratordata_.toReturn)
 #define setExpansionSuffix(coleiter, offset) ((coleiter)->iteratordata_.toReturn = (coleiter)->iteratordata_.CEpos - leftoverces)
 
-#define UCA_DATA_TYPE "dat"
+#define UCA_DATA_TYPE "icu"
 #define UCA_DATA_NAME "ucadata"
-#define INVC_DATA_TYPE "dat"
+#define INVC_DATA_TYPE "icu"
 #define INVC_DATA_NAME "invuca"
 
 /* This is an enum that lists magic special byte values from the fractional UCA */
diff --git a/icu4c/source/test/cintltst/udatatst.c b/icu4c/source/test/cintltst/udatatst.c
index c2eecb3a2c3..a6ad92e48c5 100644
--- a/icu4c/source/test/cintltst/udatatst.c
+++ b/icu4c/source/test/cintltst/udatatst.c
@@ -79,13 +79,13 @@ static void TestUDataOpen(){
     UDataMemory *result;
     UErrorCode status=U_ZERO_ERROR;
     const char* memMap[][2]={
-        {"tz", "dat"},
+        {"tz", "icu"},
         {"cnvalias", "icu"},
-        {"unames",   "dat"},
+        {"unames",   "icu"},
         {"ibm-1141", "cnv"}
     };
     const char* name           = "test";
-    const char* type           = "dat";
+    const char* type           = "icu";
     const char  dirSepString[] = {U_FILE_SEP_CHAR, 0};
 
     char* path=(char*)malloc(sizeof(char) * (strlen(ctest_dataOutDir())
@@ -160,7 +160,7 @@ static void TestUDataOpen(){
     strcat(icuDataFilePath, dirSepString);
     strcat(icuDataFilePath, U_ICUDATA_NAME);
     strcat(icuDataFilePath, "_");
-    strcat(icuDataFilePath, "tz.dat");
+    strcat(icuDataFilePath, "tz.icu");
 
     /* lots_of_mallocs(); */
     if (stat(icuDataFilePath, &stat_buf) == 0)
@@ -267,8 +267,6 @@ static void TestUDataSetAppData(){
 
     size_t            i;
        
-
-    /** srl REVISIT **/
     /* Open the testdata.dat file, using normal   */
     const char* tdrelativepath = loadTestData(&status);
     char* filePath=(char*)malloc(sizeof(char) * (strlen(tdrelativepath) + strlen(".dat") +1 +strlen(tdrelativepath)) );
@@ -447,7 +445,7 @@ static void TestUDataOpenChoiceDemo1() {
         "unames",
         "test"
     };
-    const char* type="dat";
+    const char* type="icu";
     const char* testPath="testdata";
 
     result=udata_openChoice(NULL, "icu", name[0], isAcceptable1, NULL, &status);
@@ -518,7 +516,7 @@ static void TestUDataOpenChoiceDemo2() {
     int p=2;
 
     const char* name="test";
-    const char* type="dat";
+    const char* type="icu";
     const char* path = loadTestData(&status);
 
     result=udata_openChoice(path, type, name, isAcceptable, &p, &status);
@@ -575,11 +573,11 @@ static void TestUDataGetInfo() {
     UErrorCode status=U_ZERO_ERROR;
     const char* name="cnvalias";
     const char* name2="test";
-    const char* type="dat";
+    const char* type="icu";
 
     const char* testPath=loadTestData(&status);
 
-    log_verbose("Testing udata_getInfo() for cnvalias.dat\n");
+    log_verbose("Testing udata_getInfo() for cnvalias.icu\n");
     result=udata_open(NULL, "icu", name, &status);
     if(U_FAILURE(status)){
         log_err("FAIL: udata_open() failed for path = NULL, name=%s, type=%s, \n errorcode=%s\n",  name, type, myErrorName(status));
@@ -603,7 +601,7 @@ static void TestUDataGetInfo() {
     udata_close(result);
 
 
-    log_verbose("Testing udata_getInfo() for test.dat\n");
+    log_verbose("Testing udata_getInfo() for test.icu\n");
     result=udata_open(testPath, type, name2, &status);
     if(U_FAILURE(status)) {
        log_err("FAIL: udata_open() failed for path=%s name2=%s, type=%s, \n errorcode=%s\n", testPath, name2, type, myErrorName(status));
@@ -642,7 +640,7 @@ static void TestUDataGetMemory() {
     const char* testPath = loadTestData(&status);
 
     type="icu";
-    log_verbose("Testing udata_getMemory for \"cnvalias.dat()\"\n");
+    log_verbose("Testing udata_getMemory() for \"cnvalias.icu\"\n");
     result=udata_openChoice(NULL, type, name, isAcceptable1, NULL, &status);
     if(U_FAILURE(status)){
         log_err("FAIL: udata_openChoice() failed for name=%s, type=%s, \n errorcode=%s\n", name, type, myErrorName(status));
@@ -656,8 +654,8 @@ static void TestUDataGetMemory() {
 
     udata_close(result);
 
-    type="dat";
-    log_verbose("Testing udata_getMemory for \"test.dat\"()\n");
+    type="icu";
+    log_verbose("Testing udata_getMemory for \"test.icu\"()\n");
     result=udata_openChoice(testPath, type, name2, isAcceptable3, NULL, &status);
     if(U_FAILURE(status)){
         log_err("FAIL: udata_openChoice() failed for path=%s name=%s, type=%s, \n errorcode=%s\n", testPath, name2, type, myErrorName(status));
@@ -692,7 +690,7 @@ static void TestErrorConditions(){
     };
 
     const char* name = "test";
-    const char* type="dat";
+    const char* type="icu";
 
     const char *testPath = loadTestData(&status);