The morphology of Acrochaetium dasyae Collins is described in detail from material collected along the Dutch coast. This material consists of predominantly unisexual gametophytes with a prominent persistent basal spore and a multicellular filamentous prostrate system, which may be more or less endophytic in Dasya pedicellata (C. Ag.) C. Ag„ Some additional remarks are made on field-collected specimens of A. dasyae from Swedish and Eastern- Canadian coasts. The life-history was completed in culture and turned out to be a diplobiontic cycle consisting of slightly heteromorphic generations. Gametophyte and tetrasporophyte differ in persistence or non-persistence of the basal spore, and to a lesser degree in morphology of the basal system and arrangement of the monosporangia. Material presumably representing the tetrasporophyte was found in two Dutch and several Canadian samples and provisionally identified as Audouinella saviana (Menegh.) Woelkerling; it occurs as an epiphyte on several macroalgae and on Zostera sp.. No decision could yet be made concerning the possible conspecificity of A. dasyae with several other species, described either as gametophytes or tetrasporophytes.