Chemotaxis of sperm is an important step toward fertilization. During chemotaxis, sperm change their swimming behavior in a gradient of the chemoattractant that is released by the eggs, and finally sperm accumulate near the eggs. A well established model to study chemotaxis is the sea urchin Arbacia punctulata . Resact, the chemoattractant of Arbacia , is a peptide that binds to a receptor guanylyl cyclase. The signaling pathway underlying chemotaxis is still poorly understood. Stimulation of sperm with resact induces a variety of cellular events, including a rise in intracellular pH (pH i ) and an influx of Ca 2+ ; the Ca 2+ entry is essential for the chemotactic behavior. Previous studies proposed that the influx of Ca 2+ is initiated by the rise in pH i . According to this proposal, a cGMP-induced hyperpolarization activates a voltage-dependent Na + /H + exchanger that expels H + from the cell. Because some aspects of the proposed signaling pathway are inconsistent with recent results (Kaupp, U.B., J. Solzin, J.E. Brown, A. Helbig, V. Hagen, M. Beyermann, E. Hildebrand, and I. Weyand. 2003. Nat . Cell Biol . 5:109–117), we reexamined the role of protons in chemotaxis of sperm using kinetic measurements of the changes in pH i and intracellular Ca 2+ concentration. We show that for physiological concentrations of resact (<25 pM), the influx of Ca 2+ precedes the rise in pH i . Moreover, buffering of pH i completely abolishes the resact-induced pH i signal, but leaves the Ca 2+ signal and the chemotactic motor response unaffected. We conclude that an elevation of pH i is required neither to open Ca 2+ -permeable channels nor to control the chemotactic behavior. Intracellular release of cGMP from a caged compound does not cause an increase in pH i , indicating that the rise in pH i is induced by cellular events unrelated to cGMP itself, but probably triggered by the consumption and subsequent replenishment of GTP. These results show that the resact-induced rise in pH i is not an obligatory step in sperm chemotactic signaling. A rise in pH i is also not required for peptide-induced Ca 2+ entry into sperm of the sea urchin Strongylocentrotus purpuratus. Speract, a peptide of S. purpuratus may act as a chemoattractant as well or may serve functions other than chemotaxis.