The pathogenicity of 6 isolates of Phytophthora clandestina on seedlings of 4 cultivars of subterranean clover was studied. There was a highly significant isolate x cultivar interaction in pathogenicity tests on axenic seedlings and seedlings grown in pasteurised potting mix or untreated sandy loam soil, indicating the existence of race-cultivar specificity. The isolates showed differences in virulence against the cultivars. Three isolates (race 0) caused severe disease only on Woogenellup; 2 isolates (race 1) caused severe disease on Larisa, Trikkala, and Woogenellup, but not on Meteora; one isolate (race 2) caused severe disease on Meteora and Woogenellup, but not on Larisa and Trikkala. As well as differing in virulence (the ability of a race to attack a range of cultivars), the races also differed in their aggressiveness on Woogenellup, with race 2 being the most, and race 0 the least, pathogenic. The isolates varied in their growth rate on agar medium, but this was not related to virulence or aggressiveness.