Zinc, when added to the media in concentrations as low as one micromolar, decreased protein phosphorylation in the isolated sperm head membranes in Spisula solidissima, 70-75%. Other divalent cations barium, strontium, cobalt, and copper, in similar concentrations exerted no effect. Sodium fluoride, a phosphatase inhibitor, had no effect on the zinc induced inhibition on protein phosphorylation and, when zinc was added to the media after the phosphorylation reaction of proteins, the inhibitory effect of zinc was lost. The autoradiography of -32P labeled proteins on SDS gel confirmed the inhibitory effect of zinc on protein phosphorylation. The inhibitory role of zinc on protein phosphorylation in the sperm membranes suggests a physiological role of this cation on membrane activity in the process of fertilization.