A survey was undertaken, firstly, to study the distribution of Phytophthora clandestina in the high rainfall, highly leached soils in the cool climate region of the lower south-west of Western Australia and, second, to determine the incidence of an undescribed Aphanomyces sp. in these same subterranean clover-based (Trifolium subterraneum) pastures as the occurrence of Aphanomyces in Western Australia has never been studied. P. clandestina and an Aphanomyces sp. were both frequently detected in this survey. P. clandestina was detected from 16.1% and the Aphanomyces sp. was detected from 11.4% of the plants sampled. Of the 44 locations sampled, P. clandestina was detected from 71 root samples from across 22 locations, while the Aphanomyces sp. was detected from 50 samples taken from across 23 locations. There were 31 locations from where at least one of the pathogens was detected, of which 14 yielded both pathogens. It is noteworthy that there were eight samples across six locations from where both pathogens were detected in the roots of the same individual plant. Although P. clandestina and the Aphanomyces sp. were detected in a similar number of samples, their distribution patterns were different. P. clandestina was distributed evenly across the area surveyed. In contrast, the Aphanomyces sp. was not detected across the Torbay, Albany and Napier districts.