Monospore differentiation and release in Smithora naiadum (Bangiophycidae) was found to involve a number of specialized subcellular activities. Concomitant with a rounding of the protoplast and loss of vacuolation in the vegetative cell, is the accumulation of two products originating from dictyosomes. The possible functions of these products are discussed in relation to spore release and attachment. Additional fine structural features in the developing monospore include an increased number of mitochondria and nuclear pores, a large amount of endoplasmic reticulum, and an association between the chloroplast and nuclear envelopes. Upon release, the monospore lacks a cell wall and is typified by a general lack of free cytoplasm due to the extensive accumulation of dictyosome product. In addition, the chloroplast lobes exhibit a granum-like arrangement of thylakoids.