SYNOPSIS. Sperm chemotaxis in ascidians, first demonstrated in the genera Ciona and Styela, has also been found in the genera Ascidia, Halocynthia, Chelyosoma, Pyura, Corella and Boltenia. Species—specificity exists in many cases, but unlike the Cnidaria, low level cross-specificity is more general. This is particularly true in the stolidobranchs. The sperm attractant of Ciona is a small molecule that is negatively charged and stable to heat and pronase; it binds avidly to glass surfaces. Calcium is required for sperm chemotaxis of both ascidian and cnidarian sperm. The mechanism by which turning occurs may involve both contraction of the pericentriolar process arms to shift the axis of the fiagellum relative to that of the head, and generation of asymmetric waves in the fiagellum. Hydrozoan and ascidian sperm show aspects of both behaviors, but the cnidarian sperm, known to possess a large asymmetric pericentriolar process, shows less shift of the flagelluni axis and more dramatic fiagellar asymmetry than the ascidian sperm, where an extensive pericentriolar complex has yet to be demonstrated. These differences in flagellar behavior may be adaptations to species' differences in the shape of the sperm head. They appear to prolong the turning maneuver, in contrast to the sudden turning seen in some unicellular flagellates. More data on flagellar behavior in ascidian sperm will be a valuable tool for the elucidation of the mechanism of chemotactic turning.