Others in the audience are more complimentary. 'I don't mind the coarse language - that's characteristic of the genre,' says Wang Jian, a 31-year-old financial analyst from Beijing. 'When my son gets older, I'll take him to see it. He should get some knowledge about sex - China is too strait-laced about it.'

Er ren zhuan, which originated in the rural northeast about 300 years ago, has long entertained farmers and labourers who can't afford the pricier alternatives. It's irreverent, bawdy humour, and the performers, usually a man and a woman, take aim at everything from government policies to sexual high jinks.

However, the authorities frown on such jokes as being too risque for wider audiences. Two years ago, China Central Television stopped the live telecast of one act in mid-performance, rousing heated debate about the folk art. Since then, the government has launched a drive to clean up er ren zhuan, or turn it 'green'. 'Er ren zhuan has some dirty elements in its roots,' says Sun Xijun, arts chief at the Jilin Provincial Cultural Bureau. 'It's our responsibility to get rid of this filth and make sure [the folk art] develops the right way.'

Although the government is cracking down on unlicensed troupes and unauthorised shows, it's also trying to raise the quality of the art form by promoting sanctioned performances, including free shows in the countryside, Sun says.

Private troupes fear that the push will cause the genre to lose its flavour and appeal. 'It may be widely scorned, but it's also a real reflection of life at the grass-roots,' says Xu Kaiquan, general manager of the Peace Theatre. 'How can you expect farmers to appreciate it after you change it into 'spring snow' [melodies of the elite]?'

Much of the biting satire deals with corruption, political power struggles and the widening gap between rich and poor. Among the theatre's standard acts is a ballad about corruption - alleged smuggling kingpin Lai Changxing, former Shenyang vice-mayor Ma Xiangdong and the recent pension scandal in Shanghai have all been targets. But censorship concerns have meant that the ballad is being heard less frequently, Xu says.

For all his objections to official controls, Xu has introduced plenty of his own to ensure that his performers don't step too far out of line. One device he uses is a hidden warning light on stage. If the duty manager hears inappropriate dialogue, the red light flashes. The performers are fined or even expelled if they receive repeated warnings. Managers and actors also meet every second day to discuss permissible content. A list of taboo phrases and subjects is updated from time to time. The Falun Gong movement and its leader Li Hongzhi are among the banned topics. 'We also warn our performers not to talk too much about politics now,' Xu says.

Even so, it's hard to find a balance between meeting audiences' appetite for blue humour and the government's decree for 'green' content. 'Er ren zhuan doesn't have any colour. Judgments of being blue or green are made by men,' he says. 'It's hard to make a standard judgment. Everyone has his own yardstick.'

To avoid trouble, performers joke about less sensitive topics, including international affairs, the Japanese, US President George W. Bush, Chinese emperors and the latest national foes. 'We're playing at the edge, both in terms of politics and sex,' Xu says.

Cultural commentators have diverging views on censoring the comedy. Zhao Chunqiang, deputy editor of Arts Criticisms, lauds the government's 'greening' efforts. 'Obscenity is a serious problem with er ren zhuan,' he says. 'Without control, it can become a crisis for the genre.'

However, art critic Shi Junfeng argues against official interference, saying the entertainment form will gradually evolve into being less bawdy. 'Once we get used to the blue humour, it will gradually lose its appeal. Authentic er ren zhuan, which draws raw material from real lives, can help push the boundaries of what's considered acceptable entertainment,' he says. 'But politicians don't like it because some jokes touch a raw nerve. They're using the label of indecency to shut it up.'

Reflecting the genre's roots, many performers come from poor families. At an er ren zhuan training school run by the Peace Theatre - the only one of its kind, according to Xu - more than 70 per cent of the 200 students are from the countryside. 'Only the poor can endure the rigorous training,' Xu says.

'Many skills are hard to master,' says Yuan Liangliang, a 16-year-old trainee who has calloused fingers from playing rhythmic clappers. 'I've had to work hard since I joined.'

Yuan says the 4,500 yuan annual tuition for the two-year programme is a heavy burden for his family. But he knows the investment will pay off if he becomes a contracted actor at the theatre. An actor can earn up to 1,500 yuan a month, more than 10 times the minimum cost of living in Jilin. And top talents who branch into film and television - comedian Zhao Benshan is one - can have lucrative careers.

Xu takes a fatherly interest in his performers' welfare, helping with matters such as family disputes and financial problems. 'We're like Chinese gypsies,' he says. 'Emotional bonds are often more effective than legal ones.'

Keeping the material fresh can be a problem. Many jokes and stories are modernised versions of yarns that have been passed down through the generations. Authors of original material are hard to pinpoint and copyright protection is near impossible, which means jokes that Xu comes up with can easily be appropriated by other performers.

But Xu seems unconcerned. He has signed up 50 writers, composers and directors to create new work as well as improve familiar one-liners. 'The absence of copyright protection is bad, but it also means everyone can contribute something new to the old content,' he says. 'As long as there's always something new in our performances, I won't worry about the future.'