Carcasses provide an important resource for assessing the vulnerability of bat species and sexes to threats, but the reliability of sex data derived from the external morphology (sexmorph) of bat carcasses remains uncertain. We used genetic‐based assessment of sex (sexgen) to evaluate the effect of carcass age and searcher identity on sexmorph‐based assessments of eastern red (Lasiurus borealis) and hoary (Lasiurus cinereus) bat carcasses identified by 15 different searchers at a wind‐energy facility. The proportion of carcasses for which sexmorph was unknown increased from 0.11 for those recovered within a day of death, to 0.56 within 2–3 days of death, and to ≥0.82 at ≥4 days after death. The proportion of carcasses for which sexmorph was correct decreased from 0.9 for those recovered within a day of death, to 0.65 within 2–3 days of death, and to 0.25 at ≥4 days after death. The proportion of sexmorph misidentifications of the 108 fresh carcasses (collected within 24 hours of death) varied (0.0–0.43) among searchers. These results suggest that sexmorph‐based assessments should be limited to fresh carcasses. Furthermore, additional training of people who collect and identify bat carcasses from renewable‐energy facilities may improve the accuracy of sexmorph data obtained from carcasses.