Inherent and environmental variability of the diagnostic characters of Pythium irregulare and related species was investigated. Considerable variation in the extent to which the oospore fills the oogonium and in the presence of projections on the oogonia was encountered among 25 isolates of P. irregulare studied. Sexual structures developing within the medium and at 33[degree] C had significantly fewer oogonia with projections. Numerous diclinous antheridia typical of P. debaryanum were also observed in isolates of P. irregulare grown at 33[degree] C. Diagnostic characters of P. irregulare were unaffected by various nutrient media, temperatures in the range of 18-27[degree] C, and different light conditions.