Summary. Both excess and insufficient calcium ions in the medium adversely affected the swimming performance of sea urchin spermatozoa. Sperm cells suspended in calcium-free artificial sea water swam at less than half the speed of those suspended in artificial sea water containing the optimum of 9 mmol CaCl2/1. Excess Ca2+ added to natural sea water decreased motility by 20 % at 36 mm (four times the normal amount) while the same amount of CaCl2 in artificial sea water depressed motility by 45%. The chelating agent, EGTA, caused greater depression than equivalent amounts of EDTA, though at millimolar concentration or higher both caused complete cessation of motility with the flagella in a rigidly extended position. If LaCl3 was added to artificial sea water containing 9 mm-CaCl2, it caused an even greater depression of motility. The effects were detectable at 50 nmol LaCl3/1 and complete inhibition occurred at 20 μmol/1. The flagella ceased beating in a coiled seemingly `relaxed' state. Sperm motility appears to require maintenance of Ca2+ gradients, reversible sequestration and release of Ca2+, and Ca2+-dependent metabolic regulation of the flagellar contractile system.