Commensurate with the decline in many crustacean populations has been an accumulation in reports of sexually ambiguous individuals within these populations. The cause of gynandromorphism or intersex among crustaceans is unknown. We show that gynandromorphism in the branchiopod crustacean Daphnia magna is initiated by the sex-determining hormone methyl farnesoate when levels of the hormone are intermediate between low levels that stimulate the production of broods containing all female offspring and high levels that stimulate the production of broods of all male offspring. The incidence of hormonally-induced gynandromorphism was low (0.14% at the maximum stimulatory hormone concentrations) but was significantly increased (46-fold) when the animals were hormone-treated at 30 degrees C. Some environmental chemicals also can stimulate the gynandromorphic phenotype as we demonstrated with the insecticide pyriproxyfen. Gynandromorphism occurs due to inadequate signaling of male-sex determination since: a) gynandromorphs did not occur in a population that was producing only female offspring; and, b) conditions that stimulated gynandromorphism also reduced the incidence of male offspring. We suggest that male sex determination normally occurs prior to the first embryonic cleavage. Elevated temperature may alter the timing of sex determination such that methyl farnesoate signaling occurs after the first embryonic cleavage and bilateral gynandromorphism occurs as a consequence of signaling to only one of the daughter cells. These results demonstrate that environmental factors can cause aberrant sex determination via perturbations in methyl farnesoate signaling.