In a survey of Phytophthora species associated with forest decline in Spain, Poland and France, we found three Pythium isolates, which have been characterized with internal transcribed spacer rRNA gene sequences and with classical morphological descriptors for Pythium spp. These isolates showed unique internal transcribed spacer sequences, different enough from those of any described species to justify new species status. These three distinct isolates failed to produce any sex organs with an entirely asexual reproduction and were found to represent a new species for which the name Pythium sterilum is proposed. This paper describes and illustrates the morphology of P. sterilum and presents its taxonomic position and relationships with other, related Pythium species belonging to clade K.