No single mechanism has been explicitly linked to explain global amphibian declines, but pesticides have been implicated as potential drivers. In mesocosm experiments, we tested the hypotheses that American toads (Bufo americanus) would respond differently to the insecticide carbaryl at low or high density (i.e., competition) and that postmetamorphic terrestrial growth and survival would differ with larval exposure to pesticides and competition. Carbaryl reduced survival to metamorphosis and increased time to metamorphosis in the aquatic phase, which could have a negative impact on fitness. However, carbaryl exposure increased the mass at metamorphosis of toads from high-density ponds, which may positively affect fitness. Pond periphyton showed a short-term increase in abundance after carbaryl exposure, but long-term periphyton abundance was lower than controls. No latent effects of carbaryl were seen on toad overwinter survival or mass at spring emergence in the terrestrial phase, suggesting that initial size differences at metamorphosis were overcome. Following individuals through multiple life stages is important because latent effects of exposure may not be predictable based on metamorphic endpoints alone.