The ability to distinguish sex using secondary rather than primary sexual characteristics allows the reduction or elimination of handling organisms, an advantage with rare and/or fragile species. Based on observations of differences in the morphology of gnathopods 1 and 2 of male and female cave amphipods, Gammarus acherondytes and Gammarus troglophilus, we tested the hypothesis that gnathopod morphology could be used to identify sex. To examine the size at which gnathopod metrics allowed reliable identification of sex and to establish predictive relationships, we recorded sex, total body length, and the length and width of propodi from museum collections. In an attempt to test these predictive relationships with an independent data set, we discovered that relationships were species and population specific, even after accounting for shrinkage in the preserved samples. Sex could be most successfully distinguished for G. acherondytes from Reverse Stream Cave using propodus 1 length, while for G. troglophilus it was propodus 2 length. Because regression lines of relationships diverged for G. troglophilus from Fogelpole Cave, all metrics except propodus 1 length were predictive. Although gnathopod dimorphism was limited to large individuals, it was much faster than examining individuals for the presence of penal papillae or oöstegites. Thus, using gnathopod morphology to determine sex may be adequate for some studies.