diff --git a/flang/include/flang/Parser/characters.h b/flang/include/flang/Parser/characters.h
--- a/flang/include/flang/Parser/characters.h
+++ b/flang/include/flang/Parser/characters.h
@@ -151,6 +151,33 @@
   }
 }
 
+// Does not include spaces or line ending characters.
+inline constexpr bool IsValidFortranTokenCharacter(char ch) {
+  switch (ch) {
+  case '"':
+  case '%':
+  case '\'':
+  case '(':
+  case ')':
+  case '*':
+  case '+':
+  case ',':
+  case '-':
+  case '.':
+  case '/':
+  case ':':
+  case ';':
+  case '<':
+  case '=':
+  case '>':
+  case '[':
+  case ']':
+    return true;
+  default:
+    return IsLegalIdentifierStart(ch) || IsDecimalDigit(ch);
+  }
+}
+
 struct EncodedCharacter {
   static constexpr int maxEncodingBytes{6};
   char buffer[maxEncodingBytes];
diff --git a/flang/lib/Parser/prescan.cpp b/flang/lib/Parser/prescan.cpp
--- a/flang/lib/Parser/prescan.cpp
+++ b/flang/lib/Parser/prescan.cpp
@@ -184,7 +184,8 @@
     case LineClassification::Kind::PreprocessorDirective:
       Say(preprocessed->GetProvenanceRange(),
           "Preprocessed line resembles a preprocessor directive"_en_US);
-      preprocessed->ToLowerCase().Emit(cooked_);
+      preprocessed->ToLowerCase().CheckBadFortranCharacters(messages_).Emit(
+          cooked_);
       break;
     case LineClassification::Kind::CompilerDirective:
       if (preprocessed->HasRedundantBlanks()) {
@@ -193,7 +194,9 @@
       NormalizeCompilerDirectiveCommentMarker(*preprocessed);
       preprocessed->ToLowerCase();
       SourceFormChange(preprocessed->ToString());
-      preprocessed->ClipComment(true /* skip first ! */).Emit(cooked_);
+      preprocessed->ClipComment(true /* skip first ! */)
+          .CheckBadFortranCharacters(messages_)
+          .Emit(cooked_);
       break;
     case LineClassification::Kind::Source:
       if (inFixedForm_) {
@@ -205,7 +208,10 @@
           preprocessed->RemoveRedundantBlanks();
         }
       }
-      preprocessed->ToLowerCase().ClipComment().Emit(cooked_);
+      preprocessed->ToLowerCase()
+          .ClipComment()
+          .CheckBadFortranCharacters(messages_)
+          .Emit(cooked_);
       break;
     }
   } else {
@@ -213,7 +219,7 @@
     if (line.kind == LineClassification::Kind::CompilerDirective) {
       SourceFormChange(tokens.ToString());
     }
-    tokens.Emit(cooked_);
+    tokens.CheckBadFortranCharacters(messages_).Emit(cooked_);
   }
   if (omitNewline_) {
     omitNewline_ = false;
diff --git a/flang/lib/Parser/token-sequence.h b/flang/lib/Parser/token-sequence.h
--- a/flang/lib/Parser/token-sequence.h
+++ b/flang/lib/Parser/token-sequence.h
@@ -27,6 +27,8 @@
 
 namespace Fortran::parser {
 
+class Messages;
+
 // Buffers a contiguous sequence of characters that has been partitioned into
 // a sequence of preprocessing tokens with provenances.
 class TokenSequence {
@@ -115,6 +117,7 @@
   TokenSequence &RemoveBlanks(std::size_t firstChar = 0);
   TokenSequence &RemoveRedundantBlanks(std::size_t firstChar = 0);
   TokenSequence &ClipComment(bool skipFirst = false);
+  const TokenSequence &CheckBadFortranCharacters(Messages &) const;
   void Emit(CookedSource &) const;
   void Dump(llvm::raw_ostream &) const;
 
diff --git a/flang/lib/Parser/token-sequence.cpp b/flang/lib/Parser/token-sequence.cpp
--- a/flang/lib/Parser/token-sequence.cpp
+++ b/flang/lib/Parser/token-sequence.cpp
@@ -8,6 +8,7 @@
 
 #include "token-sequence.h"
 #include "flang/Parser/characters.h"
+#include "flang/Parser/message.h"
 #include "llvm/Support/raw_ostream.h"
 
 namespace Fortran::parser {
@@ -310,4 +311,25 @@
 ProvenanceRange TokenSequence::GetProvenanceRange() const {
   return GetIntervalProvenanceRange(0, start_.size());
 }
+
+const TokenSequence &TokenSequence::CheckBadFortranCharacters(
+    Messages &messages) const {
+  std::size_t tokens{SizeInTokens()};
+  for (std::size_t j{0}; j < tokens; ++j) {
+    CharBlock token{TokenAt(j)};
+    char ch{token.FirstNonBlank()};
+    if (ch != ' ' && !IsValidFortranTokenCharacter(ch)) {
+      if (ch == '!' && j == 0) {
+        // allow in !dir$
+      } else if (ch < ' ' || ch >= '\x7f') {
+        messages.Say(GetTokenProvenanceRange(j),
+            "bad character (0x%02x) in Fortran token"_err_en_US, ch & 0xff);
+      } else {
+        messages.Say(GetTokenProvenanceRange(j),
+            "bad character ('%c') in Fortran token"_err_en_US, ch);
+      }
+    }
+  }
+  return *this;
+}
 } // namespace Fortran::parser
diff --git a/flang/test/Preprocessing/pp130.F90 b/flang/test/Preprocessing/pp130.F90
--- a/flang/test/Preprocessing/pp130.F90
+++ b/flang/test/Preprocessing/pp130.F90
@@ -1,5 +1,5 @@
-! RUN: %f18 -E %s 2>&1 | FileCheck %s
-! CHECK: j = j + &
+! RUN: (%f18 -E %s 2>&1 || true) | FileCheck %s
+! CHECK: error: bad character ('&') in Fortran token
 ! #define KWM &, use for continuation w/o pasting (ifort and nag seem to continue #define)
 #define KWM &