Endocrine disrupting chemicals that activate the estrogen receptor are routinely detected in the environment and are a concern for the health of both exposed humans and indigenous wildlife. We exposed the western clawed frog (Xenopus tropicalis) to the weak estrogen octylphenol from Nieuwkoop-Faber (NF) stage 46 tadpoles through adulthood in order to document the effects of a weak estrogen on the life history of an amphibian species. Frogs were exposed to 1, 3.3, 11 and 36 μg/L octylphenol in a continuous flow-through water system. Just prior to completion of metamorphosis (NF 65), a random subsample of froglets was collected and assessed, while the remaining frogs received continued exposure through 31 weeks of exposure when the remaining animals were sampled. Significant induction of the female egg yolk protein precursor vitellogenin was observed in the high treatment at the larval subsampling for both males and females, but not at the final sampling for either sex. No significant deviation from the control sex ratio was observed for either sampling period, suggesting minimal to no effect of octylphenol exposure on gonad differentiation. No effects in the adult frogs were observed for mortality, body mass and size, liver somatic index, estradiol and testosterone serum levels, sperm counts, or oocyte counts. The development and growth of oviducts, a female-specific secondary sex characteristic, was observed in males exposed to octylphenol. These results indicate that octylphenol exposure can induce vitellogenin in immature froglets and the development of oviducts in male adult frogs. The lack of effect observed on the developing gonads suggests that in amphibians, secondary sex characteristics are more susceptible to impact from estrogenic compounds than the developing gonads.