Pollution and the introduction of nonnative predators, typically fish, are two frequent human-associated stressors in freshwater ecosystems. Amphibians appear to be particularly susceptible to these stressors. We conducted a mesocosm experiment to examine the independent and interactive effects of an invasive fish, the Western Mosquitofish (Gambusia affinis), and ammonium nitrate, an agricultural fertilizer, on American Toad (Anaxyrus americanus) tadpoles. Mosquitofish tended to have a negative effect on American Toad survivorship. American Toad metamorphs from mesocosms with mosquitofish were smaller than those from mesocosms without mosquitofish. Mosquitofish also delayed metamorphosis of the American Toads. Ammonium nitrate addition did not affect survivorship. However, American Toad metamorphs from ammonium nitrate addition treatments were smaller than those from treatments without ammonium nitrate addition. Mosquitofish and ammonium nitrate addition each had independent effects on American Toads, but there was no evidence for significant interactions between these two stressors.