Sea urchin sperm motility is modulated by sperm-activating peptides. One such peptide, speract, induces changes in intracellular free calcium concentration ([Ca 2+ ] i ). High resolution imaging of single sperm reveals that speract-induced changes in [Ca 2+ ] i have a complex spatiotemporal structure. [Ca 2+ ] i increases arise in the tail as periodic oscillations; [Ca 2+ ] i increases in the sperm head lag those in the tail and appear to result from the summation of the tail signal transduction events. The period depends on speract concentration. Infrequent spontaneous [Ca 2+ ] i transients were also seen in the tail of unstimulated sperm, again with the head lagging the tail. Speract-induced fluctuations were sensitive to membrane potential and calcium channel blockers, and were potentiated by niflumic acid, an anion channel blocker. 3-isobutyl-1-methylxanthine, which potentiates the cGMP/cAMP-signaling pathways, abolished the [Ca 2+ ] i fluctuations in the tail, leading to a very delayed and sustained [Ca 2+ ] i increase in the head. These data point to a model in which a messenger generated periodically in the tail diffuses to the head. Sperm are highly polarized cells. Our results indicate that a clear understanding of the link between [Ca 2+ ] i and sperm motility will only be gained by analysis of [Ca 2+ ] i signals at the level of the single sperm.