Embryos and larvae of the South African frog Xenopus laevis were exposed to ammonium perchlorate (AP) or control medium for 70 d. The dosage levels (59 ppb, 14,140 ppb) bracketed a range of perchlorate concentrations measured in surface waters at the Longhorn Army Ammunition Plant (LHAAP) in Karnack, Texas, USA. The experiment also included a 28-d nontreatment recovery period to assess the reversibility of AP effects. There were no significant effects of AP on mortality or hatching success. There were no effects of AP on developmental abnormalities such as bent/asymmetric tails or edema. Ammonium perchlorate inhibited forelimb emergence, the percentage of animals completing tail resorption, and hindlimb development during the 70-d exposure period. Only the upper AP concentration reduced whole-body thyroxine content, whereas both concentrations caused significant hypertrophy of the thyroid follicular epithelium. Both concentrations of AP caused a skewed sex ratio, significantly reducing the percentage of males at metamorphosis. The effects of AP on metamorphosis and thyroid function were reversed during the 28-d nontreatment recovery period. We conclude that AP inhibits thyroid activity and alters gonadal differentiation in developing X. laevis. These effects were observed at concentrations at or below concentrations reported in surface waters contaminated with ammonium perchlorate, suggesting that this contaminant may pose a threat to normal development and growth in natural amphibian populations.