diff --git a/com.google.eclipse.protobuf.test/src/com/google/eclipse/protobuf/validation/ProtobufJavaValidator_checkFieldModifiers_Test.java b/com.google.eclipse.protobuf.test/src/com/google/eclipse/protobuf/validation/ProtobufJavaValidator_checkFieldModifiers_Test.java
index c8c87a7..e620d16 100644
--- a/com.google.eclipse.protobuf.test/src/com/google/eclipse/protobuf/validation/ProtobufJavaValidator_checkFieldModifiers_Test.java
+++ b/com.google.eclipse.protobuf.test/src/com/google/eclipse/protobuf/validation/ProtobufJavaValidator_checkFieldModifiers_Test.java
@@ -13,8 +13,10 @@
 import static com.google.eclipse.protobuf.protobuf.ProtobufPackage.Literals.MESSAGE_FIELD__MODIFIER;
 import static com.google.eclipse.protobuf.validation.Messages.mapWithModifier;
 import static com.google.eclipse.protobuf.validation.Messages.missingModifier;
+import static com.google.eclipse.protobuf.validation.Messages.oneofFieldWithModifier;
 import static com.google.eclipse.protobuf.validation.ProtobufJavaValidator.MAP_WITH_MODIFIER_ERROR;
 import static com.google.eclipse.protobuf.validation.ProtobufJavaValidator.MISSING_MODIFIER_ERROR;
+import static com.google.eclipse.protobuf.validation.ProtobufJavaValidator.ONEOF_FIELD_WITH_MODIFIER_ERROR;
 import static org.eclipse.xtext.validation.ValidationMessageAcceptor.INSIGNIFICANT_INDEX;
 import static org.mockito.Mockito.mock;
 import static org.mockito.Mockito.verify;
@@ -112,4 +114,33 @@
     validator.checkFieldModifiers(field);
     verifyZeroInteractions(messageAcceptor);
   }
+
+  // syntax = "proto2";
+  //
+  // message Foo {
+  //   oneof test {
+  //     string bar = 1;
+  //     string baz = 2;
+  //   }
+  // }
+  @Test public void should_not_create_error_if_no_modifier_on_oneof_field() {
+    MessageField field = xtext.find("bar", MessageField.class);
+    validator.checkFieldModifiers(field);
+    verifyZeroInteractions(messageAcceptor);
+  }
+
+  // syntax = "proto2";
+  //
+  // message Foo {
+  //   oneof test {
+  //     repeated string bar = 1;
+  //     string baz = 2;
+  //   }
+  // }
+  @Test public void should_create_error_if_modifier_on_oneof_field() {
+    MessageField field = xtext.find("bar", MessageField.class);
+    validator.checkFieldModifiers(field);
+    verify(messageAcceptor).acceptError(oneofFieldWithModifier, field, MESSAGE_FIELD__MODIFIER,
+        INSIGNIFICANT_INDEX, ONEOF_FIELD_WITH_MODIFIER_ERROR);
+  }
 }
diff --git a/com.google.eclipse.protobuf/src/com/google/eclipse/protobuf/Protobuf.xtext b/com.google.eclipse.protobuf/src/com/google/eclipse/protobuf/Protobuf.xtext
index 3e0ce2a..6c60868 100644
--- a/com.google.eclipse.protobuf/src/com/google/eclipse/protobuf/Protobuf.xtext
+++ b/com.google.eclipse.protobuf/src/com/google/eclipse/protobuf/Protobuf.xtext
@@ -81,7 +81,7 @@
   '}' (';')?;
 
 OneOfElement returns MessageElement:
-  Extensions | Group;
+  Extensions | Group | MessageField;
 
 Extensions:
   ->'extensions' ranges+=Range (',' ranges+=Range)* (';')+;
diff --git a/com.google.eclipse.protobuf/src/com/google/eclipse/protobuf/validation/Messages.java b/com.google.eclipse.protobuf/src/com/google/eclipse/protobuf/validation/Messages.java
index 8b6c645..8f80459 100644
--- a/com.google.eclipse.protobuf/src/com/google/eclipse/protobuf/validation/Messages.java
+++ b/com.google.eclipse.protobuf/src/com/google/eclipse/protobuf/validation/Messages.java
@@ -34,6 +34,7 @@
   public static String missingFieldNumber;
   public static String missingModifier;
   public static String multiplePackages;
+  public static String oneofFieldWithModifier;
   public static String requiredInProto3;
   public static String scopingError;
   public static String unknownSyntax;
diff --git a/com.google.eclipse.protobuf/src/com/google/eclipse/protobuf/validation/Messages.properties b/com.google.eclipse.protobuf/src/com/google/eclipse/protobuf/validation/Messages.properties
index df4f28f..9c15670 100644
--- a/com.google.eclipse.protobuf/src/com/google/eclipse/protobuf/validation/Messages.properties
+++ b/com.google.eclipse.protobuf/src/com/google/eclipse/protobuf/validation/Messages.properties
@@ -17,6 +17,7 @@
 mapWithModifier = Field labels (required/optional/repeated) are not allowed on map fields.
 missingFieldNumber = Missing field number.
 missingModifier = Missing modifier: \"required\", \"optional\", or \"repeated\".
+oneofFieldWithModifier = Fields inside "oneof" cannot have a modifier.
 multiplePackages = Multiple package definitions.
 requiredInProto3 = Required fields are not allowed in proto3.
 scopingError = It may be caused by an imported non-proto2 file.
diff --git a/com.google.eclipse.protobuf/src/com/google/eclipse/protobuf/validation/ProtobufJavaValidator.java b/com.google.eclipse.protobuf/src/com/google/eclipse/protobuf/validation/ProtobufJavaValidator.java
index adbb50d..3ec975b 100644
--- a/com.google.eclipse.protobuf/src/com/google/eclipse/protobuf/validation/ProtobufJavaValidator.java
+++ b/com.google.eclipse.protobuf/src/com/google/eclipse/protobuf/validation/ProtobufJavaValidator.java
@@ -22,6 +22,7 @@
 import static com.google.eclipse.protobuf.validation.Messages.mapWithModifier;
 import static com.google.eclipse.protobuf.validation.Messages.missingModifier;
 import static com.google.eclipse.protobuf.validation.Messages.multiplePackages;
+import static com.google.eclipse.protobuf.validation.Messages.oneofFieldWithModifier;
 import static com.google.eclipse.protobuf.validation.Messages.requiredInProto3;
 import static com.google.eclipse.protobuf.validation.Messages.unknownSyntax;
 import static com.google.eclipse.protobuf.validation.Messages.unrecognizedSyntaxIdentifier;
@@ -67,6 +68,7 @@
   public static final String REQUIRED_IN_PROTO3_ERROR = "requiredInProto3";
   public static final String INVALID_MAP_KEY_TYPE_ERROR = "invalidMapKeyType";
   public static final String MAP_WITH_MAP_VALUE_TYPE_ERROR = "mapWithMapValueType";
+  public static final String ONEOF_FIELD_WITH_MODIFIER_ERROR = "oneofFieldWithModifier";
 
   @Inject private IndexedElements indexedElements;
   @Inject private NameResolver nameResolver;
@@ -109,6 +111,10 @@
       checkMapField(field);
       return;
     }
+    if (field.eContainer() instanceof OneOf) {
+      checkOneOfField(field);
+      return;
+    }
     if (field.getModifier() == ModifierEnum.UNSPECIFIED && isProto2Field(field)) {
       error(missingModifier, field, MESSAGE_FIELD__MODIFIER, MISSING_MODIFIER_ERROR);
     } else if (field.getModifier() == ModifierEnum.REQUIRED && isProto3Field(field)) {
@@ -123,6 +129,13 @@
     }
   }
 
+  private void checkOneOfField(MessageField field) {
+    if (field.getModifier() != ModifierEnum.UNSPECIFIED) {
+      error(oneofFieldWithModifier, field, MESSAGE_FIELD__MODIFIER,
+          ONEOF_FIELD_WITH_MODIFIER_ERROR);
+    }
+  }
+
   private boolean isProto2Field(MessageField field) {
     EObject container = field.eContainer();
     if (container != null && !(container instanceof Protobuf)) {