We used patch.use theory, giving.up densities in experimental food patches, and harvest.rate measurements within these patches to determine the relative contributions of predation risk and energy to foraging costs in four species of rodents from communities in the Sonoran and Negev deserts. To partition costs into components of energy and predation, we converted field measurements of giving.up densities into harvest rates (J min"1), used these harvest rates as an estimate of total foraging costs, estimated energetic foraging costs from published physiological measurements of activity and thermoregulatory costs, and assumed that missed opportunity costs were either zero or negative. Our results showed that predation costs predominate. Energetic costs represented only 24%, 19%, 16% and 13% of the foraging costs for Merriam's kangaroo rat (Dipodomys merriami; Sonoran), the round.tailed ground squirrel (Spermophilus tereticaudus; Sonoran), the greater Egyptian sand gerbil (Gerbillus pyramidum; Negev), and Allenby's gerbil (G. allenbyi; Negev), respectively. Equally important were predation.risk differences between bush and open microhabitats; the microhabitat differences in predation cost were often 2.4 times larger than the animals' energetic costs. Seasonal patterns in foraging costs also were predominantly influenced by predation rather than energetic costs. Predation costs appear to be greater in the Negev Desert, but rodents of the Sonoran desert experience greater seasonal and microhabitat variability in predation costs. As a result, predation risk may contribute more towards species coexistence in the community of the Sonoran Desert than that of the Negev Desert.