Mechanisms resulting in parapatric distributions of closely related taxa have long interested ecologists. If two species are distributed across an environmental gradient, and differ in their ability to cope with environmental conditions, the outcome of competitive interactions may be dependent on prevailing abiotic conditions. Two closely related species of poison frogs in north-central Peru were observed to occupy parapatric distributions across an elevation gradient. Ameerega bassleri is a highland endemic restricted to a small region of Peru, and A. trivittata is distributed throughout lowland Amazonia. The goal of this study was to examine the effect of an abiotic factor (elevation) on two biotic factors (intraspecific and interspecific competition), by measuring growth and survival in the larvae of A. trivittata and A. bassleri. Using mesocosm experiments arranged in a fractional factorial design, we found that (1) A. bassleri had a strong negative effect on the growth and survival of A. trivittata regardless of elevation, (2) A. trivittata had no effect on the growth of A. bassleri at either elevation, but did appear to reduce the survival of A. bassleri more strongly in the lowlands than highlands, (3) lowland conditions uniformly reduced survival in all treatments and in both species, and (4) competition was strongly asymmetric between A. bassleri and A. trivittata. We conclude that the perceived low density of A. trivittata in highland sites may be influenced by the presence of A. bassleri, but the lower limit to the distribution of A. bassleri cannot be explained by competition with A. trivittata and may be due to physiological constraints imposed by lowland conditions. © 2008 The Authors.