Polyacrylam~ de gel electrophoresis revealed three genetic polymorphisms among central California populations of the supralittoral copepod Tigriopus californicus. Laboratory analysis of mated pairs and their progeny confirmed the allelic nature of esterase (EST), phosphoglucose isomerase (PGI), and leucine aminopeptidase (LAP) electromorphs. Polymorphism within each enzyme system was localized such that only a few of the nine or more sites sampled were polymorphic while the others were fixed on the same allele. While the Pescadero site (located near the middle of the 250 km transect studied) was polymorphic for PG1 with two alleles at 0.5 frequency, one of these forms never reached a frequency of 0.04 or higher in any of the other populations sampled, including a population located only 1.5 km to the south. Similar population differentiation was observed with respect to the EST locus, and to a lesser extent, the LAP locus. EST phenotype frequencies at Moss Beach exhibited both microgeographic and temporal variation. No obvious patterns, however, were observed among the phenotype frequencies, and estimated allele frequencies indicate that remarkable consistency existed among all the'Moss Beach population samples. These data indicate that T. californicuspopulations located within habitat patches are genetically relatively homogeneous, while populations occupying habitats isolated by stretches of sandy beach can show strong genetic differentiation over short geographic distances.