Nonylphenol (NP) is a persistent, lipophilic, and toxic chemical that can be endocrine disrupting (estrogenic) at sublethal concentrations. Since amphibian metamorphosis is a hormone-driven process and a delicate balance of hormone levels is required for successful metamorphosis, exposure of larval amphibians to NP might disrupt metamorphic processes. This study tested whether NP exposure influenced rate of metamorphic progression and tail resorption in bullfrog (Rana catesbeiana) tadpoles. Premetamorphic bullfrog tadpoles were exposed for 7 d to one of 3 nominal concentrations of NP (234 μg/L, 468 μg/L, or 936 μg/L) with or without the addition of exogenous 3,3',5-triiodothyronine (T₃). In the absence of exogenous T₃, NP significantly increased the rate of tail growth (as measured by tail length) at 936 μg/L. There was no significant effect of NP alone on tail width, limb development, or the process of cranial transformation. When T₃ was added to the treatments, increasing NP concentrations were associated with a significant decrease in the rate of cranial transformation, and at the highest dose, the rate of tail resorption was significantly lower than in the controls. Overall, NP had an inhibitory effect on the rate of bullfrog tadpole metamorphic progression and tail resorption.