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Some aquatic taxa (fishes, snakes) exhibit a puzzling form of sexual dimorphism: males have a more rugose body surface than do conspecific females. As a first step towards understanding the biological significance of this phenomenon, the nature and correlates (sex, size, body condition, season, and latitude) of scale rugosity was quantified in preserved museum specimens of four species of sea snakes (Family Hydrophiidae): Astrotia stokesii, Emydocephalus annulatus, Hydrophis elegans, and Lapemis hardwickii. Males are more rugose than conspecific females in all four taxa, on ventral as well as dorsal scales. The position of rugosities on the body differs among species. For example, rugosities are found over the entire body in E. annulatus, but are concentrated anteriorly in L. hardwickii and posteriorly in H. elegans. Females possess rugosities that are similar to those of conspecific males, but smaller (in female E. annulatus, they are visible only with scanning electron microscopy analysis). The degree of male rugosity varies seasonally in at least two species (E. annulatus, L. hardwickii), being most pronounced during the winter breeding season. Thus, the transition from terrestrial to aquatic life in proteroglyphous snakes has been accompanied both by an increase in overall rugosity, and by a seasonally labile sex-specific elaboration of this trait. © 2006 The Linnean Society of London, Biological Journal of the Linnean Society, 2006, 89, 343–354.