Sex identification of many species of adult riffle beetles (Coleoptera: Elmidae) has relied historically upon removal and examination of genitalia, which leads to death if examined individuals are initially alive. This process and its consequences present a substantial problem if investigators intend to study live beetles in captivity, or if an investigator is examining population sex ratios of threatened or at-risk beetles in the wild where destructive sampling should be avoided. We examined external morphological characteristics of three species of Heterelmis Sharp [H. comalensis (Bosse, Tuff, and Brown, 1988), Heterelmis cf. glabra (Horn), and Heterelmis vulnerata (LeConte)] and developed a non-lethal method of identifying sexes by using these characters. Across species, several external morphological characteristics differed between the sexes, but the length of the fifth abdominal ventrite was the most reliable and robust external character found to vary between the sexes. However, sex misidentification error rates using proposed species-specific length thresholds for the fifth ventrite differed among the three Heterelmis species. This easily applied technique has utility for identifying the sexes of the three Heterelmis species in studies that require live subjects and/or minimal mortality, such as in life history studies or observations of sex ratios in the field.