We examined how condition (weight at length) of rainbow troutOncorhynchus mykissvaried in relation to availability of drifting invertebrates and temperature at two locations over four seasons in the Green River, Utah. Food availability (daytime drift density) varied more than 17-fold across sites and seasons, and rainbow trout experienced an 11°C range in mean monthly temperature. Both rainbow trout gut fullness and condition increased as joint, nonlinear functions of increasing food availability and increasing temperature. Variation in condition decreased with fish size, although condition of intermediate sized fish was most strongly related to variation in food and temperature. Observed relationships between rainbow trout condition, drift abundance, and temperature were qualitatively consistent with bioenergetic models that predict fish growth should vary as a joint function of food and temperature. However, the inferred temperature optima for rainbow trout seemed to be significantly higher than predicted. Our results support a growing body of evidence that stream trout may be frequently food-limited in nature.