Determining the mechanisms responsible for the success of invasive species is critical for developing effective management strategies. Artificially draining managed wetlands to maintain natural ephemeral conditions is a common practice in the Pacific Northwest and is assumed to kill invasive American bullfrog (Lithobates catesbeianus) larvae, which typically overwinter in permanent wetlands before metamorphosis. Bullfrogs in the Willamette Valley, Oregon, however, have invaded ephemeral wetland sites with confirmed metamorphosis within 4 months after hatching at 1 site. We hypothesized that plasticity in growth and development rates in response to hydroperiod facilitated bullfrog invasion in Oregon. We tested this hypothesis by quantifying larval bullfrog development and growth in response to 3 hydroperiod conditions in a mesocosm setting. We tested clutches collected from both ephemeral (n = 3) and permanent (n = 3) wetlands. We found no differences in development or growth due to hydroperiod treatments (body length, P = 0.48; mass, P = 0.27), but we found differences in growth among clutches (P ≤ 0.001). These differences likely represent natural variation in growth rates because clutches collected from the same wetland type did not respond with similar growth and geographic barriers between collection sites did not account for the differences. These results indicate a lack of plasticity to hydroperiod and suggest that artificial hydroperiod manipulation in the Pacific Northwest will not induce rapid metamorphosis by invasive bullfrog larvae, although some genotypes may be capable of rapid growth and metamorphosis. © 2013 The Wildlife Society.