diff --git a/mypy/fastparse.py b/mypy/fastparse.py
index 68925edb7682..c5ae01558ded 100644
--- a/mypy/fastparse.py
+++ b/mypy/fastparse.py
@@ -733,6 +733,9 @@ def is_star2arg(k: ast35.keyword) -> bool:
     # Num(object n) -- a number as a PyObject.
     @with_line
     def visit_Num(self, n: ast35.Num) -> Union[IntExpr, FloatExpr, ComplexExpr]:
+        if getattr(n, 'underscores', None) and self.pyversion < (3, 6):
+            raise FastParserError('Underscores in numeric literals are only '
+                                  'supported in Python 3.6', n.lineno, n.col_offset)
         if isinstance(n.n, int):
             return IntExpr(n.n)
         elif isinstance(n.n, float):
@@ -904,3 +907,7 @@ def __init__(self, msg: str, lineno: int, offset: int) -> None:
         self.msg = msg
         self.lineno = lineno
         self.offset = offset
+
+
+class FastParserError(TypeCommentParseError):
+    pass
diff --git a/mypy/test/testcheck.py b/mypy/test/testcheck.py
index 209076ecddf2..bb92f2e22445 100644
--- a/mypy/test/testcheck.py
+++ b/mypy/test/testcheck.py
@@ -72,6 +72,9 @@
 if 'annotation' in typed_ast.ast35.Assign._fields:
     files.append('check-newsyntax.test')
 
+if 'underscores' in typed_ast.ast35.Num._fields:
+    files.append('check-underscores.test')
+
 
 class TypeCheckSuite(DataSuite):
     def __init__(self, *, update_data=False):
diff --git a/test-data/unit/check-underscores.test b/test-data/unit/check-underscores.test
new file mode 100644
index 000000000000..51926c3adf1e
--- /dev/null
+++ b/test-data/unit/check-underscores.test
@@ -0,0 +1,15 @@
+[case testUnderscoresRequire36]
+# flags: --fast-parser --python-version 3.5
+x = 1000_000  # E: Underscores in numeric literals are only supported in Python 3.6
+[out]
+
+[case testUnderscoresSyntaxError]
+# flags: --fast-parser --python-version 3.6
+x = 1000_000_  # E: invalid token
+[out]
+
+[case testUnderscoresBasics]
+# flags: --fast-parser --python-version 3.6
+x: int
+x = 1000_000
+y: str = 1000_000.000_001  # E: Incompatible types in assignment (expression has type "float", variable has type "str")