The definition of Ectocarpus by Hamel (1931–39) is accepted and his division of the genus into two species complexes, E. fasciculati and E. siliculosi, vindicated. Observations on the E. fasciculatus complex show that its component taxa rep-resent various stages in thallus growth and elaboration, while the subspecific taxa of E. siliculosus are determined primarily by size and shape of plurilocular sporangia. Both species are shown to undergo a streblonemoid phase in their early development. Field observations are confirmed by following the growth of both species in culture. The habitat and host range of both species is found to be wide.