Mechanisms that maintain species isolation within sympatric congeners have attracted analysis in many kinds of organisms, but not in snakes. We studied two sibling species of amphibious sea snakes (Laticauda colubrina and L. frontalis) on the island of Efate, in the Pacific Ocean republic of Vanuatu. The two taxa are almost identical morphologically, except that L. colubrina grows much larger than L. frontalis. No natural hybrids have been reported, and geographic distributions of the two taxa suggest the possibility of sympatric speciation. Our fieldwork shows that the two taxa are often syntopic and overlap in breeding seasons. Behavioral studies in outdoor arenas show that the separation between these two taxa is maintained by species-specific cues that control male courtship. Males of both species courted conspecific females but not heterospecific females. The proximate mechanism driving this separation involves chemical cues. Adult females of both taxa possess distinctive lipids in the skin. Males directed courtship behavior (chin-pressing) to hexane-extracted samples of lipids from conspecific but not heterospecific females. Males of the dwarf species (L. frontalis) were more selective courters than were those of the larger taxon (L. colubrina), perhaps because a preference for courting larger females means that L. colubrina males would be unlikely to court L. frontalis–sized (i.e., small) females even in the absence of pheromonal barriers.