We investigated the effects of potassium, sodium, glucose, and calcium concentrations, alone or in combinations, on sperm motility in muskellungeEsox masquinongy. Sperm motility was evaluated by the duration of sperm movement and the initial percentage of motile sperm. The osmolality of diluents rather than the specific ions or nonelectrolyte played a major role in the regulation of sperm motility in muskellunge. Sperm were fully activated (>80%) when the concentration was lower than 50 mM of KCl and NaCl, or 100 mM glucose (all in 30 mM tris–HCl at pH 8.0). A small percent of spermatozoa could be activated at 150 mM KCl and NaCl, or 300 mM glucose, which were hypertonic to the seminal plasma. The duration of sperm movement was up to 6–7 min at 12°C in a solution of 100 mM glucose or 50 mM NaCl. Spermatozoa had a prolonged duration of movement in potassium solutions, up to 120 min at 12°C in a solution of 100 mM KCl. The prolonged duration of movement might be caused by reactivation of sperm or gradual activation of sperm motility. Calcium had an inhibitory effect on sperm motility in muskellunge, starting at 3 mM CaCl2with 30 mM tris–HCl at pH 8.0. Semen diluted in calcium-supplemented solutions did not disperse well, and the sperm tended to form clumps. The mechanism involved in muskellunge sperm motility control markedly differs from that in salmonids (inhibitory function of K+and activatory role of Ca2+) and cyprinids (no effect of Ca2+).