A stem rot of broccoli caused by a species of Pythium was found at Tsu city, Mie Prefecture in October 1993. The infected stems appeared white rot with aqueous edge. The morphology of sexual organs made by the isolate closely fits van der Plaats-Niterink's description of Pythium ultimum Trow var. ultimum. The isolate was pathogenic to seedlings of many crop species. This is the first report on stem rot of broccoli caused by Pythium ultimum Trow var. ultimum in Japan.