Gotta catch 'em all: that's the Pokemon slogan. And this boy of 13 has made a killing out of it

Kevin Cox is a dealer. The 13-year-old schoolboy is one of a growing number of teenagers earning a living by exploiting the black market.

Some youngsters may dabble in drugs, but Kevin has found another lucrative sideline. He trades in rare, highly profitable Pokémon cards, featuring the mini-monster characters with super powers, which have flooded British playgrounds.

Kevin, a pupil at Lilian Baylis school in south London, buys random packs of 11 cards for £2.50 each and sells those featuring the rarest monsters for up to £15. He has sold two cards of the Charizard character, which he got in the same pack, for £9 each.

"I first discovered Pokémon when I saw my seven-year-old brother Troy with it," he said. "So I started to collect the cards and sell them to other people. I buy them off kids who don't want them and sell them on to primary school children for profit.

"I have made a lot of money out of it. They are very easy to sell. You just buy them, pick out the ones with the star on the corner and sell them for up to £15," Kevin told the television show Nick News, to be shown this evening on satellite channel Nickelodeon. "I don't even know how to play the game."

The idea that a 13-year-old can demand profits that would impress a drug dealer for pieces of laminated card seems obscene. But such is Pokémon's hold on the pre-pubescent; the game is marketed so effectively that it makes previous playground phenomena look ephemeral. Its latest manifestation, Pokémon: The First Movie (note that "first"), opens today across the UK.

Pokémon was created as a Gameboy computer game in 1995 by Satoshi Tajiri, a Japanese inventor, who based the characters on his childhood experiences of collecting small creatures. Five years later, Squirtle, Butterfree and Jigglypuff may not mean much to an adult, but they, and their 148 brightly coloured siblings, have spawned a £6bn industry, selling 12 million copies in Japan, 2 million in America and 500,000 here since the game's launch in Britain in October.

Pokémon is no longer just a game: it is a film, a television show, trading cards, dozens of toys and nearly 1,000 other licensed products. So collectable are the cards that their "secret" factory in east London is closely guarded. In the US, two factories have been prosecuted for Pokémon forgery.

Parents talk of having to "drag" their children to school from Sky television's Pokémon cartoons, screened between 8am and 9am, and again at 4pm when they return home. The frenzy is unlikely to subside; as well as the long-awaited film, six more Pokémon games are set for release before Christmas.

Still, childhood passions are often swiftly spent. There can be few children's beds under which have not lurked discarded Furbies, Power Rangers or Cabbage Patch Dolls. As publisher Dorling Kindersley has found, as it counts the cost of unsold Star Wars merchandise worth millions of pounds, most crazes, no matter how frenetic, end just as violently. But Pokémon's appeal has proven far more enduring - and experts say its strategy is likely to be the benchmark for toy marketing. Merchandise has been strategically released via a drip-drip method that ensures children always have something to hanker after.

This week, said Antonella Kendrick, mother of nine-year-old Emanuele, it is the new cards that go with the film. "Everywhere we go - on the television, to the supermarket, the newsagent - Pokémon is there. Today we went to the video store in Cambridge and it was all the new video games."

Emanuele said: "We talk about it all the time at school, and all the new things that have just come out. There's a lot of competition to get things."

The competition is no accident. Wizards of the Coast, Pokémon's manufacturers, have 70 instructors who visit toyshops giving lessons. They have organised roadshows that have attracted thousands of children, helping to fuel the frenzy. Pokémon is carefully contrived to appeal both to the gaming and collecting instincts, and to both sexes. Alongside the fighting, there is nurturing and teaching. With the weapons come big eyes and sweet faces.

Tim Burke, author of Saturday Morning Fever: Growing Up With Cartoon Culture, has described Pokémon as "a cultural phenomenon, not just a toy fad". He said: "It's an amazing piece of alchemy, the way one tie-in has seamlessly led to another, and the way each has intensified the entire happening. Pokémon has set a new standard - it's inescapable, it's everywhere."

But it has its detractors. Parents are ambivalent about creatures that teach moral responsibility and maths on one hand, and greed and gambling on the other. In America, a 12-year-old boy was convicted of stealing cards at knifepoint, and two mothers were charged with disorderly conduct after a row over children's cards.

Some schools here have banned them. Edinburgh Academy Preparatory School headmaster Campbell Paterson said: "Pupils who usually made the most of playtime were standing around in huddles exchanging cards. [They] were not concentrating in the classroom because their minds were on their next Pokémon swap."

In Birmingham, Moseley CE Junior school banned them for a different reason. Dr Julia Burton, head teacher, said a pupil had been upset by a boy's mother. "He thought she was going to call the police because he hadn't done a fair swap."

Wizards of the Coast aims to discourage such behaviour, holding organised swap sessions. Samantha Ward, UK product manager for Pokémon, said: "If somebody wants to sell a card there is nothing we can do about it, but we try to ensure the children are aware they can get the cards free by trading another card. That's the beauty of the concept of the game."

But as the Pokémon slogan "Gotta catch 'em all" suggests, there is constant pressure to keep on buying. Soon to be launched here are Pokémon Snap and Pokémon Stadium, and yet more characters.

Yet this may prove its undoing. Guy Sanders, 10, from Saffron Walden in Essex (100-odd Pokémon cards, two games) thinks Pokémon will last "until the end of summer. I'll probably get bored of them eventually." The next thing, he thinks, "might be mini-skateboards". And what was his last craze? He has already forgotten.