Examination of genotypes of female-pup nursing pairs taken from large maternity colonies of the Mexican free-tailed bat in Texas demonstrates that nursing is nonrandom and selective along genetic (kinship) lines. This is contrary to previous reports that nursing in these colonies is indiscriminate. Although nursing is nonrandom, an estimated 17 percent of the females sampled were nursing pups that could not be their offspring. This "nonparental" nursing is an apparent result of the difficulties females face in consistently relocating and selectively nursing their own pups within these enormous colonies.