In Crustacea, the dominant pattern of sequential hermaphroditism is protandry (sex change from male to female). Here we provide the first evidence from external morphology and population structure that Gnorimosphaeroma oregonense, an abundant, sexually dimorphic intertidal isopod, undergoes protogynous (female to male) sex change. In the field, 31% of females had rudimentary penes, suggesting sex change and laboratory growth experiments confirmed that females produced one brood of juveniles, then passed through a variable number of molts as immature males before becoming sexually mature males. Contrary to reports for other protogynous Crustacea, this study suggests that in G. oregonense sex change is not socially mediated, although it may be facultative, because a large percentage of laboratory-reared juvenile isopods developed directly into males. Potential adaptive explanations for protogyny are discussed in relation to protandry--the more common strategy in Crustacea.