Have you heard the one about the Stasi officer?

BY DAY Wolfgang Kolditz marshals the flow of leaflets and Biros marked "Proud to be Gay" from the offices of an Aids charity in seedy Kreuzberg. By night he dresses up in uniform and gatecrashes parties, invades cabaret floors and television studios.

His entrance never fails to turn heads. "Germans are impressed by uniforms," Mr Kolditz says. Especially by his. For at night Mr Kolditz becomes "Colonel Jorg Weigang, head of Department 34 of the Stasi, in charge of jokes, speeches, rumours and special duties".

The Stasi were East Germany's dreaded security police, worthy successors of the Gestapo. Ten years after their demise, the sight of a Stasi uniform can still put the fear of God into East Germans, though memories have faded enough for Mr Kolditz to escape instant lynching. After the initial shock, hilarity ensues, especially when Wolfgang, alias Col Weigang, starts telling his jokes.

Funnily enough, it is the Stasi's former victims who tend to laugh most at the gag, while the people who never suffered at the Communist secret police's hands remain unmoved. In east Berlin Col Weigang brings the house down, in the west the audience stares in disbelief. To Mr Kolditz, a 59- year-old East German, that is evidence of yet another Wessi peculiarity.

He still recalls with a shudder the night he spent 45 minutes telling jokes on the stage of the Deutsche Oper in west Berlin without provoking a single smile. The next act, concentrating exclusively on one-liners about bodily functions, had the audience in stitches.

"You notice that western Germans have little understanding of humour," Mr Kolditz says. "They laugh only at pornographic or scatological jokes." In the east humour was used as a well-honed weapon against the Communist regime - or, encouraged by the Stasi, served as a safety valve for resentment, according to conspiracy theorists. Mr Kolditz has found no evidence that the authorities ran a secret joke-making factory, but has come across many former Stasi officers who are delighted with his act. One of them, a certain Colonel Hans, was so grateful he lent the comedian the uniform he now wears during his performances. Mr Kolditz must return it only when the real colonel, aged 70, dies. He wants to be buried in it.

Whether the Stasi were responsible for the jokes or not, nearly five decades of Communism has produced a good crop of them, which Mr Kolditz is trying to preserve for posterity. When not goose-stepping at a party, the impersonator spends his time collecting East Germany's humour output. On Wednesday night, the last time he counted them, his tally stood at 16,403, including, admittedly, some Wessi imports.

There was a barren time, naturally, in the months of euphoria that followed the fall of the Wall. But when the first East German companies started going under, the anonymous wags' creative juices began to flow afresh. There was once again an enemy, someone to lampoon: Wessis.

Anti-Wessi jokes form the bulk of Mr Kolditz's repertoire, so it is hardly surprising that audiences in the west are not always amused. Some, though, are double-edged. "Why are Chinese people always smiling?" runs one. Answer: "Because they still have their Wall." Others are harder to appreciate on the other side of the mental divide: "What do clouds and Wessis have in common?" "Everything becomes fine when they disappear."

These and similar gems are to be published next year under the title "Wessis must also die". Originally, Mr Kolditz's collection was to be called "Thank God that Wessis die", but the publisher thought that was too offensive.

The quality of fresh jokes, Mr Kolditz concedes, is declining, and he strongly disapproves of new trends. "I hear a lot of anti-Semitic jokes at the moment," he says. "It is a sign of the times. Ordinary people don't understand why, 55 years after the war, Germany must pay another DM10bn in a new Holocaust settlement."

He does not collect this kind of joke - the punch-line usually features gas chambers - though they would have certainly worked well on one particular outing. He was in the west, parading in a uniform similar to those one worn by Prussian officers. The crowd, he says were aged 70 and over, their eyes turning misty when he marched in to the tune of "Prussia's Glory", and nodding off as he started on his one-liners.

Only afterwards did he discover that he had been entertaining a reunion of the Waffen SS. "I did have some qualms," he admits.