diff --git a/doc/ca65.sgml b/doc/ca65.sgml
index 03aa72097..a40bcddff 100644
--- a/doc/ca65.sgml
+++ b/doc/ca65.sgml
@@ -2316,6 +2316,24 @@ Here's a list of all control commands and a description, what they do:
   </verb></tscreen>
 
 
+<sect1><tt>.ISMNEM, .ISMNEMONIC</tt><label id=".ISMNEMONIC"><p>
+
+  Builtin function. The function expects an identifier as argument in braces.
+  The argument is evaluated, and the function yields "true" if the identifier
+  is defined as an instruction mnemonic that is recognized by the assembler.
+  Example:
+
+  <tscreen><verb>
+  	.if  	.not .ismnemonic(ina)
+  	  	.macro ina
+  	  	  	clc
+  	  	  	adc #$01
+  	  	.endmacro
+  	.endif
+
+  </verb></tscreen>
+
+
 <sect1><tt>.DESTRUCTOR</tt><label id=".DESTRUCTOR"><p>
 
   Export a symbol and mark it as a module destructor. This may be used
diff --git a/src/ca65/expr.c b/src/ca65/expr.c
index 8b87d0860..c4da32824 100644
--- a/src/ca65/expr.c
+++ b/src/ca65/expr.c
@@ -62,6 +62,7 @@
 #include "symtab.h"
 #include "toklist.h"
 #include "ulabel.h"
+#include "macro.h"
 
 
 
@@ -417,6 +418,34 @@ static ExprNode* FuncDefined (void)
 
 
 
+static ExprNode* FuncIsMnemonic (void)
+/* Handle the .ISMNEMONIC, .ISMNEM builtin function */
+{
+    int Instr = -1;
+
+    /* Check for a macro or an instruction depending on UbiquitousIdents */
+
+    if (CurTok.Tok == TOK_IDENT) {
+        if (UbiquitousIdents) {
+            /* Macros CAN be instructions, so check for them first */
+            if (FindMacro (&CurTok.SVal) == 0) {
+                Instr = FindInstruction (&CurTok.SVal);
+            }
+        } else {
+            /* Macros and symbols may NOT use the names of instructions, so just check for the instruction */
+            Instr = FindInstruction (&CurTok.SVal);
+        }
+    } else {
+        Error ("Identifier expected.");
+    }
+    /* Skip the name */
+    NextTok ();
+
+    return GenLiteralExpr (Instr > 0);
+}
+
+
+
 ExprNode* FuncHiByte (void)
 /* Handle the .HIBYTE builtin function */
 {
@@ -1065,6 +1094,10 @@ static ExprNode* Factor (void)
             N = Function (FuncDefined);
             break;
 
+        case TOK_ISMNEMONIC:
+            N = Function (FuncIsMnemonic);
+            break;
+
         case TOK_HIBYTE:
             N = Function (FuncHiByte);
             break;
diff --git a/src/ca65/pseudo.c b/src/ca65/pseudo.c
index 2e8a473a5..6d27c05c4 100644
--- a/src/ca65/pseudo.c
+++ b/src/ca65/pseudo.c
@@ -2040,6 +2040,7 @@ static CtrlDesc CtrlCmdTab [] = {
     { ccNone,           DoIncBin        },
     { ccNone,           DoInclude       },
     { ccNone,           DoInterruptor   },
+    { ccNone,           DoUnexpected    },      /* .ISMNEMONIC */
     { ccNone,           DoInvalid       },      /* .LEFT */
     { ccNone,           DoLineCont      },
     { ccNone,           DoList          },
diff --git a/src/ca65/scanner.c b/src/ca65/scanner.c
index db747cf70..30494b365 100644
--- a/src/ca65/scanner.c
+++ b/src/ca65/scanner.c
@@ -223,6 +223,8 @@ struct DotKeyword {
     { ".INCBIN",        TOK_INCBIN      },
     { ".INCLUDE",       TOK_INCLUDE     },
     { ".INTERRUPTOR",   TOK_INTERRUPTOR },
+    { ".ISMNEM",        TOK_ISMNEMONIC  },
+    { ".ISMNEMONIC",    TOK_ISMNEMONIC  },
     { ".LEFT",          TOK_LEFT        },
     { ".LINECONT",      TOK_LINECONT    },
     { ".LIST",          TOK_LIST        },
diff --git a/src/ca65/token.h b/src/ca65/token.h
index 7aaba34c3..5ffe42466 100644
--- a/src/ca65/token.h
+++ b/src/ca65/token.h
@@ -199,6 +199,7 @@ typedef enum token_t {
     TOK_INCBIN,
     TOK_INCLUDE,
     TOK_INTERRUPTOR,
+    TOK_ISMNEMONIC,
     TOK_LEFT,
     TOK_LINECONT,
     TOK_LIST,