Invasive fish frequently negatively affect amphibian populations around the world. In agricultural regions of the world, the effects of invasive fish on amphibians may coincide with pollution by agricultural fertilizers. We conducted a mesocosm experiment to examine the potential independent and interactive effects of introduced Western Mosquitofish (Gambusia affinis) and ammonium nitrate, an agricultural fertilizer, on Green Frog (Lithobates clamitans) tadpoles. Mosquitofish had a negative effect on the survivorship of Green Frog tadpoles. Green Frog tadpoles from mesocosms with mosquitofish were larger than tadpoles from mesocosms without mosquitofish, possibly due to a thinning effect that reduced intraspecific competition, as well as increased primary productivity. Ammonium nitrate additions did not affect survivorship in Green Frogs. However, Green Frog tadpoles in ammonium nitrate addition treatments were larger than those from treatments without ammonium nitrate addition, possibly due to increased phytoplankton abundance. In conclusion, mosquitofish and ammonium nitrate addition each had independent and additive effects on Green Frog tadpoles, but there was no evidence for significant interactions between these two stressors. Our results suggest that environmental stressors can have additive effects in some systems and that introduced fish predators may have greater impacts on amphibian populations than low level chemical contamination.