Computer assisted sperm analysis (CASA) was used to characterize the motility of sea bass Dicentrarchus labrax spermatozoa and to study the effect of several physicochemical variables and heavy metals on sperm swimming performance. Duration of sperm motility in sea bass was very short (< 50 s). During the first 20 s all the motility variables measured remained approximately constant, the velocity and linearity of the movement being maximum during this period, while both variables decreased sharply later. While slight variations in pH did not significantly modify sperm swimming performance, changes in osmolality affected all the measured motility variables. Two of the heavy metals tested, Cu2+ and Pb2+, did not affect sperm motility when the activating media contained up to 100 ppm of the metal salts. In contrast, Hg2+ modified the morphology of post-swimming spermatozoa at 0.4-1 ppm (sperm dilution rate 1:39) and completely arrested sperm motility at concentrations as low as 0.1 ppm (sperm dilution rate 1:2500). Assuming a covalent binding to sperm cells, this revealed a finite number of c. 10 million Hg2+ binding sites per spermatozoon. Complementary results using demembranated spermatozoa suggested that the main target of HgCl2 would be located in the plasma membrane and that HgCl2 would inhibit water channels, hence preventing sperm motility.