Natural contamination by zearalenone, a toxic metabolite of Fusarium fungi, was surveyed in 160 samples of sorghum imported from 2001 to 2006 into Japan for feed. Of these 160 samples, 84 (52.5%) were contaminated with zearalenone, ranging in concentration from 60 to 7,260 μg/kg. In the contaminated sorghum samples, F. semitectum, F. verticillioides, F. oxysporum, and other Fusarium spp. were detected. The concentration of zearalenone was well correlated with the development of colonies of F. semitectum and other Fusarium spp. When the isolates of F. semitectum and F. verticillioides were cultivated on sorghum, zearalenone was found only in F. semitectum culture. These results indicate that F. semitectum is a causal fungus of zearalenone contamination in sorghum.