Twenty-seven samples of fodder wheat from the 1980 harvest, visibly attacked by mould, were analyzed for the content of zearalenone. Zearalenone was found to be present in ten samples and its content ranged from 50 to 1300 micrograms per kg. One of the five samples of pelleted feeds, taken at random, contained 14 mg zearalenone per kg. Fifty-three moulds of the genus Fusarium were isolated from the feeds. Among the 19 moulds of F. graminearum, 15 produced zearalenone; 11 moulds of F. culmorum produced zearalenone in all cases, and of the two moulds of F. tricinctum one produced zearalenone. Ten moulds of F. oxysporum, eight moulds of F. sporotrichioides and three molds of F. moniliforme did not produce any zearalenone. Under the described conditions the tested moulds produced 200 to 1200 mg of zearalenone per 1 kg.