According to Mexico's National Association of Matadors, Michel "Michelito" Lagravere set the international record at his age for bull calves killed in a two-hour fight watched by more than 3,500 people.
But Guinness World Records saif it was not aware that the event was taking place and will not recognise the result.
"We do not accept records based on the killing or harming of animals," it said on its website.Michelito, who turned 11 in December, took the news in his stride. "It's all the same to me because in the world of bullfighting the record is now part of history, although it may not be for Guinness," he said.
Michelito's parents and the organisers of the fights in which he stars have been criticised for allowing the 4ft 5in tall fighter to risk his safety in the ring at such a young age.
Mexico has no minimum age for bullfighting, although by law children are supposed to be closely supervised as they advance from nonlethal exhibitions with small calves to full-fledged lethal bullfights.
One Spanish bullfighter, Jairo Miguel, was nearly gored to death in Mexico in 2007 at the age of 14 when a bull rushed him at high speed and punctured his lung.
A child welfare law enacted last year in Michelito's home state led authorities to briefly suspend last Saturday's landmark bullfight. Investigators allowed the show to go on, but the Yucatan state human rights commission said it still has concerns.
"It's a latent risk that he doesn't have the maturity and the physical strength to do these kinds of activities," said Guadalupe Sosa Escobedo, spokesman for the commission.
His father runs a school for matadors and his mother, Diana Peniche Marenco, said her son has "got it in his blood". But she added: "I'd be lying to you if I said I wasn't afraid."
