The ultrastructural features of oospore wall ornamentation in the genusTolypellawere 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 ofT. glomerataandT. nidifica, our results support the status of separate species. Close relationships and transitional forms may exist betweenT. nidificaandT. normaniana, and not only in oospore wall ornamentation. Oospores ofT. hispanicaexhibited the same distinct type of reticulate oospore wall as previously reported, but our results do not support the recognition ofT. hispanicaas a separate species. Ultrastructure of the oospore walls ofT. proliferaandT. intricatawas almost identical, suggesting that these species are closely related. We therefore reject previous suggestions that morphological characteristics of oospores as observed inSEMare sufficient for identification of individual species. Although significant differences were found among oospores in individual species ofTolypella, large variation among populations, and among individuals belonging to the same population, caused substantial overlap among species.