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ow nloaded from The chronic toxicity of methoxychlor to the South African clawed frog, Xenopus (Silurana) tropicalis was evaluated using a life cycle approach. The chronic exposure period ranged from mid-cell blastula stage (NF [Nieuwkoop and Faber, 1994] stage 8) to 90-d of exposure during which time the organisms generally completed metamorphosis and emerged as juvenile frogs. Methoxychlor concentrations ranged from 1 to 100 µg/L. Methoxychlor concentrations>10 µg/L caused delayed development. Organisms exposed to 10 µg/L methoxychlor for 30-d showed enlarged thyroid glands with follicular hyperplasia. No increase in mortality or external malformation was observed at any of the test concentrations during early embryo-larval development (NF stage 8 to NF stage 46; ca. 2-d exposure). A concentration-dependent increase in external malformations and internal abnormalities of the liver and gonads were noted following 90-d exposure, however. Skewing of sex ratio toward the female gender, decrease