InXiphophorus helleriseveral alleged instances of sex inversion in females were described. Later researchers, doubting such findings, regarded the species as having separate sexes, of males, strong and weak, and females, strong and weak, in relation to the differing ratios of male and female sex determiners, and they considered the alleged sex inverted specimens to be cases of arrhenoidy of old females or weak males, in which the male secondary sex characters appear late so that in the male immature stage they could be mistaken for young females.