Sea urchin eggs attract sperm through chemotactic peptides, which evoke complex changes in membrane voltage and in the concentrations of cyclic AMP, cyclic GMP and Ca2+ ions The intracellular signalling pathways and their cellular targets are largely unknown. We have now cloned, from sea urchin testis, the complementary DNA encoding a channel polypeptide, SPIH. Functional expression of SPIH gives rise to weakly K+-selective hyperpolarization-activated channels, whose activity is enhanced by the direct action of cAMP. Thus, SPIH is under the dual control of voltage and cAMP. The SPIH channel, which is confined to the sperm flagellum, may be involved in the control of flagellar beating. SPIH currents exhibit all the hallmarks of hyperpolarization-activated currents (Ih), which participate in the rhythmic firing of central neurons, control pacemaking in the heart, and curtail saturation by bright light in retinal photoreceptors. Because of their sequence and functional properties, Ih channels form a class of their own within the superfamily of voltage-gated and cyclic-nucleotide-gated channels.