Sperm dimorphism, where ejaculates contain both fertile "eusperm" and sterile "parasperm", has been implicated in sperm competition in animals. Most parasperm lack an acrosome, eliminate the nucleus and develop a complex cytoplasm, swollen with secretory vesicles. This study of parasperm in the foliate whelk Ceratostoma foliatum, focuses on characterizing the process of nuclear breakdown and the nature of cytoplasmic secretions. Testis squash preparations were treated with a Promega apoptosis detection [terminal deoxynucleotidyl transferase (TdT)-mediated dUTP nick-end labeling-TUNEL] assay (Cedar Lane) to detect nuclear apoptosis; acridine orange was used to localize lysosomes. Parasperm were isolated on Percoll gradients and their proteins characterized by running electrophoresis of lysed, purified fractions on polyacrylamide gels. Results show that the process of nuclear breakdown follows the hallmarks of apoptosis in ultrastructure, which is confirmed by the presence of 3'-OH overhangs of DNA fragments. This research implicates snail parasperm as models for elucidating the nuclear events of apoptosis.