The ultrastructure of vegetative and reproductive cells in Sporocladopsis novae-zelandiae has been studied for the first time to determine the precise taxonomic position of this genus. Vegetative cells contained one parietal chloroplast with 5-8 ulvophycean-type pyrenoids and transverse cell walls with plasmodesmata. The growth of the transverse walls was centripetal by furrow ingrowth. Ovoid zoosporangia initially had one nucleus and a large basal vacuole. Zoospore release occurred by the apical disintegration of the zoosporangial wall and expansion of an apical mucilage plug. Emptied zoosporangia could be occupied by a series of successive new zoosporangia developed from the same mother cell. Zoospores were bi- or quadriflagellate. The flagellar apparatus showed a 180 ° rotational symmetry and overlapped basal bodies with bilobed terminal caps covering their proximal ends. Our study permits us to conclude that Sporocladopsis is an ulvophycean genus. This conclusion is based on the occurrence of overlapping basal bodies, cytokinesis by infurrowing, and ulvophycean type of pyrenoids. Bilobed terminal caps and zoospore release by apical disintegration of the cell wall (which is accompanied by expansion of a mucilage plug) in Sporocladopsis suggest affinities with the order Ulvales. However, the absence of plasmodesmata and the absence of percurrent proliferation of zoosporangia in all members of the order Ulvales challenge this taxonomical position. The plasmodesmata in Sporocladopsis are similar to those in the Trentepohliales and could suggest a relationship with the latter, but the typical features of the Trentepohliales are absent in Sporocladopsis and this absence does not support an affinity between Sporocladopsis and the Trentepohliales. In conclusion, the fine structural characters of Sporocladospsis are paradoxical and it is therefore difficult to relate this genus to an order within the class Ulvophyceae. Molecular data of the genus, which are not available at present, would be necessary before making a final taxonomic decision concerning its ordinal position inside the class Ulvophyceae.