Pythium senticosum and P. takayamanum spp. nov. were isolated from cool-temperate forest soil in Japan. P. senticosum can grow at 5 C and is fast growing at 25 C with a radial growth of 22.2 mm 24 h(-1). The species is morphologically characterized by ovoid to ellipsoid sporangia with apical papilla, ornamented oogonia with acute conical spines, and antheridia with broad attachment to oogonia. P. takayamanum is very different and can grow at 35 C. This species is morphologically characterized by its wavy antheridial stalks and ellipsoidal oogonia with constricted areas. Phylogenetic analyses of the ITS rDNA region and the partial COX2 gene showed that the two species are genetically distinct from each other and from their closest relatives. P. senticosum is closely related to P. dimorphum and P. undulatum whereas P. takayamanum is closely related to P. rhizosaccharum and P. parvum.