The Japanese endemic charalean species Nitella gracilens Morioka (=N. furcata subsp. furcata var. sieberi f. gracilens) was thought to be extinct until it was rediscovered at the type locality, Lake Ashinoko, Kanagawa Prefecture. Uni-charalean cultures of N. gracilens were established and sexual organs developed and matured. The morphology of this alga, examined by light microscopy, corresponds closely with the original description by Morioka, particularly with regard to the oospores. The mature oospore walls of N. gracilens are orange or yellowish-brown and the fossa wall of the oospore is smooth or has a finely granulate ornamentation. In addition, scanning electronic microscopic examination demonstrated that the fossa walls have minute granulate structures that are compactly arranged. These observations contrast with a previous report that the oospore wall ornamentation of this taxon is reticulate. Since oospore wall ornamentation and the form of the dactyls in N. gracilens are essentially different from those of other infraspecific taxa of N. furcata sensu Wood (1965), N. gracilens should be maintained as a distinct morphological species within the genus Nitella.