We quantified the distribution and abundance of larvae of two species of caddisfly, Parapsyche elsis and Arctopsyche grandis (Trichoptera: Hydropsychidae), at 8 sites along a 560-m elevational gradient and a 36-km longitudinal gradient in a pristine mountain stream. Both species are widely distributed throughout the northern Rocky Mountains and have similar 2-year life-spans, similar catch-nets, and similar microhabitat requirements. However, the reach-scale distribution patterns were distinctly dissimilar. Parapsyche elsis larvae were most abundant in the upper reaches of the study stream, while A. grandis larvae were found only in lower stream segments. We examined the correlation between the stream gradient and the environmental variables that likely influence the distributions of these insects: temperature, food quantity and quality, current velocity, and substratum cobble size. Only those variables relating to temperature (i.e., annual degree-days, summer maximum) correlated with stream elevational and longitudinal gradients. Laboratory examination revealed a relationship between zones of temperature independence in the metabolism-temperature response of both species, maximum summer temperatures in the stream, and distribution of the two species. We propose that the physiological response of these species to stream temperature, with resulting bioenergetics, is the probable mechanism structuring the stream distribution and abundance patterns of these species.