diff --git a/pyflakes/checker.py b/pyflakes/checker.py
index c8ccf56..d157008 100644
--- a/pyflakes/checker.py
+++ b/pyflakes/checker.py
@@ -1799,6 +1799,10 @@ class Checker(object):
             addArgs(node.args.args)
             defaults = node.args.defaults
         else:
+            if PY38_PLUS:
+                for arg in node.args.posonlyargs:
+                    args.append(arg.arg)
+                    annotations.append(arg.annotation)
             for arg in node.args.args + node.args.kwonlyargs:
                 args.append(arg.arg)
                 annotations.append(arg.annotation)

diff --git a/pyflakes/test/test_type_annotations.py b/pyflakes/test/test_type_annotations.py
index 289535d..1fa4f5e 100644
--- a/pyflakes/test/test_type_annotations.py
+++ b/pyflakes/test/test_type_annotations.py
@@ -418,3 +418,11 @@ class TestTypeAnnotations(TestCase):
                 Y = 2
                 return Y
         """, m.UndefinedName)
+
+    @skipIf(version_info < (3, 8), 'new in Python 3.8')
+    def test_positional_only_argument_annotations(self):
+        self.flakes("""
+        from x import C
+
+        def f(c: C, /): ...
+        """)

F401 (imported but unused) false positive on Foo when using def x(a: Foo, /, *, b: Bar)
```python
from datetime import datetime as Foo, time as Bar


def x(a: Foo, /, *, b: Bar):
    pass
```

Running this on the file results in:

```
› flake8 --version  
3.7.9 (mccabe: 0.6.1, pycodestyle: 2.5.0, pyflakes: 2.1.1) CPython 3.8.1 on Linux
› pyflakes --version
2.1.1 Python 3.8.1 on Linux
› flake8 example.py 
example.py:1:1: F401 'datetime.datetime as Foo' imported but unused
› pyflakes example.py
example.py:1: 'datetime.datetime as Foo' imported but unused
```
pyflakes/test/test_type_annotations.py::TestTypeAnnotations::test_positional_only_argument_annotations
pyflakes/test/test_type_annotations.py::TestTypeAnnotations::test_annotated_async_def pyflakes/test/test_type_annotations.py::TestTypeAnnotations::test_not_a_typing_overload pyflakes/test/test_type_annotations.py::TestTypeAnnotations::test_overload_in_class pyflakes/test/test_type_annotations.py::TestTypeAnnotations::test_overload_with_multiple_decorators pyflakes/test/test_type_annotations.py::TestTypeAnnotations::test_postponed_annotations pyflakes/test/test_type_annotations.py::TestTypeAnnotations::test_return_annotation_is_class_scope_variable pyflakes/test/test_type_annotations.py::TestTypeAnnotations::test_return_annotation_is_function_body_variable pyflakes/test/test_type_annotations.py::TestTypeAnnotations::test_typeCommentsAdditionalComment pyflakes/test/test_type_annotations.py::TestTypeAnnotations::test_typeCommentsAssignedToPreviousNode pyflakes/test/test_type_annotations.py::TestTypeAnnotations::test_typeCommentsFullSignature pyflakes/test/test_type_annotations.py::TestTypeAnnotations::test_typeCommentsFullSignatureWithDocstring pyflakes/test/test_type_annotations.py::TestTypeAnnotations::test_typeCommentsInvalidDoesNotMarkAsUsed pyflakes/test/test_type_annotations.py::TestTypeAnnotations::test_typeCommentsMarkImportsAsUsed pyflakes/test/test_type_annotations.py::TestTypeAnnotations::test_typeCommentsNoWhitespaceAnnotation pyflakes/test/test_type_annotations.py::TestTypeAnnotations::test_typeCommentsStarArgs pyflakes/test/test_type_annotations.py::TestTypeAnnotations::test_typeCommentsSyntaxError pyflakes/test/test_type_annotations.py::TestTypeAnnotations::test_typeCommentsSyntaxErrorCorrectLine pyflakes/test/test_type_annotations.py::TestTypeAnnotations::test_typeIgnore pyflakes/test/test_type_annotations.py::TestTypeAnnotations::test_typeIgnoreBogus pyflakes/test/test_type_annotations.py::TestTypeAnnotations::test_typeIgnoreBogusUnicode pyflakes/test/test_type_annotations.py::TestTypeAnnotations::test_typingExtensionsOverload pyflakes/test/test_type_annotations.py::TestTypeAnnotations::test_typingOverload pyflakes/test/test_type_annotations.py::TestTypeAnnotations::test_typingOverloadAsync pyflakes/test/test_type_annotations.py::TestTypeAnnotations::test_variable_annotations