- We evaluated how environmental factors influence foraging costs and, consequently, the use of resource patches by two species of nocturnal, seed-eating desert rodents, Gerbillus allenbyi (Allenby's gerbil) and G. pyramidum (greater Egyptian sand gerbil), in a sand-dune habitat in the northwestern Negev Desert of Israel. To do so, we examined correlations between intensity of patch use and weather conditions and other environmental factors. In addition, we considered the influence of bush and open microhabitats on patch use. We quantified patch use by measuring the amount of millet seed reamining (giving-up densities) in experimental-resource patches during nine 7-night sampling periods between November 1986 and January 1988. Overall, we found significant positive correlations between giving-up densities and cloud cover, microhabitat, and moon phase. We also found significant negative correlations between giving-up densities and relative humidity, minimum overnight temperature, and the interaction between cloud cover and moon phase. Patch use by gerbils was most intense (giving-up densities were lowest) in the bush microhabitat and on warm, damp, and dark nights. Warm nights may represent lower metabolic costs of foraging, damp nights may represent greater ease of foraging because of heightened olfaction, and dark nights and the bush microhabitat may represent reduced predatory risk in time and space, respectively. Our results corroborate the hypothesis that patch use in gerbils results from joint considerations of energetic costs and predatory risk.