diff --git a/pytorch_translate/data/data.py b/pytorch_translate/data/data.py
index 03eba1fd..ef0fb189 100644
--- a/pytorch_translate/data/data.py
+++ b/pytorch_translate/data/data.py
@@ -261,7 +261,11 @@ def create_from_file(path, is_npz=True, num_examples_limit: Optional[int] = None
             return result
         else:
             # idx, bin format
-            return InMemoryIndexedDataset(path)
+            impl = data.indexed_dataset.infer_dataset_impl(path)
+            if impl == "mmap":
+                return data.indexed_dataset.MMapIndexedDataset(path)
+            else:
+                return InMemoryIndexedDataset(path)
 
     def subsample(self, indices):
         """
diff --git a/pytorch_translate/options.py b/pytorch_translate/options.py
index c20cfe44..c3769b98 100644
--- a/pytorch_translate/options.py
+++ b/pytorch_translate/options.py
@@ -293,6 +293,12 @@ def add_preprocessing_args(parser):
         default="",
         help="Path for the binary file containing target side monolingual data",
     )
+    group.add_argument(
+        "--fairseq-binary-data-format",
+        default=False,
+        action="store_true",
+        help="Binary data paths are prefixes of .bin and .idx files",
+    )
 
     # TODO(T43045193): Move this to multilingual_task.py eventually
     group.add_argument(
diff --git a/pytorch_translate/preprocess.py b/pytorch_translate/preprocess.py
index 6667b27e..031ae6d3 100644
--- a/pytorch_translate/preprocess.py
+++ b/pytorch_translate/preprocess.py
@@ -15,7 +15,7 @@
 from pytorch_translate.data.dictionary import Dictionary
 
 
-def maybe_generate_temp_file_path(output_path=None):
+def maybe_generate_temp_file_path(output_path=None, is_npz=True):
     """
     This function generates a temp file path if output_path is empty or None.
     This is useful to do before calling any preprocessing function that has a
@@ -28,7 +28,7 @@ def maybe_generate_temp_file_path(output_path=None):
         os.close(fd)
     # numpy silently appends this suffix if it is not present, so this ensures
     # that the correct path is returned
-    if not output_path.endswith(".npz"):
+    if is_npz and not output_path.endswith(".npz"):
         output_path += ".npz"
     return output_path
 
@@ -148,16 +148,18 @@ def preprocess_corpora(args, dictionary_cls=Dictionary):
             utils.maybe_parse_collection_argument(args.train_target_binary_path), str
         ):
             args.train_source_binary_path = maybe_generate_temp_file_path(
-                args.train_source_binary_path
+                args.train_source_binary_path,
+                is_npz=not args.fairseq_binary_data_format,
             )
             args.train_target_binary_path = maybe_generate_temp_file_path(
-                args.train_target_binary_path
+                args.train_target_binary_path,
+                is_npz=not args.fairseq_binary_data_format,
             )
     args.eval_source_binary_path = maybe_generate_temp_file_path(
-        args.eval_source_binary_path
+        args.eval_source_binary_path, is_npz=not args.fairseq_binary_data_format
     )
     args.eval_target_binary_path = maybe_generate_temp_file_path(
-        args.eval_target_binary_path
+        args.eval_target_binary_path, is_npz=not args.fairseq_binary_data_format
     )
 
     # Additional text preprocessing options could be added here before
diff --git a/pytorch_translate/train.py b/pytorch_translate/train.py
index d8a644c6..9c335407 100644
--- a/pytorch_translate/train.py
+++ b/pytorch_translate/train.py
@@ -300,6 +300,7 @@ def setup_training_model(args):
             src_bin_path=args.train_source_binary_path,
             tgt_bin_path=args.train_target_binary_path,
             weights_file=getattr(args, "train_weights_path", None),
+            is_npz=not args.fairseq_binary_data_format,
         )
 
     if args.task == "dual_learning_task":
@@ -311,6 +312,7 @@ def setup_training_model(args):
             split=args.valid_subset,
             src_bin_path=args.eval_source_binary_path,
             tgt_bin_path=args.eval_target_binary_path,
+            is_npz=not args.fairseq_binary_data_format,
         )
 
     return task, model, criterion