The ultrastructural features of oospore wall ornamentation in the genus Tolypella were examined using scanning electron microscopy (SEM). The taxonomic relationships among several species were discussed on the basis of oospore ultrastructure and measurements. In the case of T. glomerata and T. nidifica, our results support the status of separate species. Close relationships and transitional forms may exist between T. nidifica and T. normaniana, and not only in oospore wall ornamentation. Oospores of T. hispanica exhibited the same distinct type of reticulate oospore wall as previously reported, but our results do not support the recognition of T. hispanica as a separate species. Ultrastructure of the oospore walls of T. prolifera and T. intricata was almost identical, suggesting that these species are closely related. We therefore reject previous suggestions that morphological characteristics of oospores as observed in SEM are sufficient for identification of individual species. Although significant differences were found among oospores in individual species of Tolypella, large variation among populations, and among individuals belonging to the same population, caused substantial overlap among species.