The development and ultrastructure of the spermatium of Porphyra gardneri has been studied using the light and electron microscopes. The spermatia of P. gardneri and the Florideophycidae have many ultrastructural similarities. This evidence alone suggests that they may also have a similar function. Spermatium attachment to a prototrichogyne produced by the carpogonium and subsequent transfer of contents of the spermatium via a fertilization canal to the prototrichogyne and carpogonium has been observed with the light and electron microscopes. This evidence combined with the observation of a diploid chromosome number in the carpospores and vegetative cells of the conchocelis phase is considered unequivocal evidence that sexual reproduction occurs in the life history of P. gardneri.