The acrosome of Ciona intestinalis spermatozoa is a flattened vesicle, approximately 200 × 180 × 40 nm in length, width, and height, respectively. It contains moderately electron-dense material with an electron-dense plate in its center. The outer membrane of the acrosome is in close contact with the overlying plasmalemma. Neither a subacrosomal substance nor a perforatorium can be identified in the subacrosomal space. The acrosome fragments into smaller vesicles (approximately 20–30 nm in diameter) in both suspended and inseminated spermatozoa. This phenomenon has been observed in all spermatozoa examined in both cases. The sperm reaction ( C. C. Lambert and D. Epel, 1979, Dev. Biol. 69, 296–304) is not a prerequisite for the initiation of the fragmentation. The possibility is presented that the structural change in the acrosome reported here is a new type of acrosome reaction which has not been previously described. Some speculations are also presented on ascidian fertilization.