The larval stages ofAmbystoma jeffersonianumandA. maculatumcoexist throughout much of their ranges. Mesocosm studies in field pens indicate that coexistence is due to strong intraspecific density-dependent effects and microhabitat shifts associated with predator avoidance. I used laboratory experiments to determine the limiting resource causing the observed density-dependent effects for each species by separating the effects of food and space. Both species experienced reduced survivorship and altered growth rates at high density and in the presence of heterospecific larvae, confirming results from previous field pen studies. Interspecific interactions are a complex combination of intraguild predation (including cannibalism) and competition. Data from treatments with reduced food and space are consistent withA. jeffersonianumbeing space-limited andA. maculatumbeing a food-limited.