Growth inhibitors were isolated from an arctic strain of Trichoderma polysporum, and the structures were elucidated and the in vitro inhibitory effects of these compounds against Pythium iwayamai were investigated. Eleven compounds were isolated; four showed a concentration-dependent growth-inhibitory effect against P. iwayamai. None of these compounds have been reported previously as substances with antimicrobial activity against P. iwayamai. One of these four compounds inhibited the growth of the pathogen at 33 μg ml(-1) concentration during a 15-day incubation at 20 °C. This effect was comparable to that of chloroneb (1: 1,4-dichloro-2,5-dimethoxybenzene), a fungicide with activity against P. iwayamai. Thus, the results of the present study show that the arctic strain of T. polysporum can be an effective source of antibiotics with activity against the snow rot pathogen, P. iwayamai.