Amphibians face a variety of anthropogenic environmental perturbations that could act alone or in combination to influence population size. We investigated interactive effects of warming conditions, a moderate pulse of nitrogen pollution, and conspecific density on larvae of the common frog, Rana temporaria. The 16-day experiment had a 2 × 2 × 2 factorial design implemented in 80-l outdoor mesocosms. High density and warm temperature both resulted in reduced activity and visibility; tadpoles grew and developed more quickly at low density and high temperature. The high-nitrogen treatment did not influence behavior, growth, or development rate. We attribute this to several realistic features of our study, including a pulsed treatment application and natural denitrification within the mesocosms. There was only a single interaction among the three factors: higher temperature exacerbated density-dependence in growth rate. These results illustrate that climate warming may benefit temperate amphibians, although the benefits may be counteracted by enhanced larval crowding.