A wide range of cryoprotectants and cooling rates have been tested on the spermatozoa of two polychaete speciesArenicola marinaandNereis virens.The techniques were more successful withN. virens;with this species active sperm were recovered after immersion in liquid nitrogen following a variety of cooling protocols. Structural damage to the sperm of both species, however, was observed following freezing, and this was more severe in the case ofA. marina.Structural damage of the flagellum plasma membrane was most commonly observed. Biologically important changes in the sperm can be induced by the freezing procedures; for example acrosome eversion may be induced inN. virenssperm. Dissociation of the sperm ofA. marinacan be induced after freezing by the sperm activator quinacrine, but these sperm do not acquire forward motility probably due to flagellum damage.