Species in the virilis group of Drosophila (fruit flies), which overlap or replace one another along climatic gradients, exhibit corresponding differences in basal thermotolerance, inducible thermotolerance and the heat-shock response. The low-latitude species D. virilis exceeds the high-latitude species D. lummei in these measures of thermotolerance, the temperature threshold for heat-shock factor (HSF) activation and the ability to express hsp70 mRNA and diverse heat-shock proteins (e.g. Hsp70, Hsp83 and small Hsps) after intense heat shock (e.g. 40-41 degrees C). The xeric species D. novamexicana differs from the mesic species D. texana in much the same way for many of these traits. By contrast, intraspecific variation in these traits is small. Because D. virilis and D. lummei can readily be crossed to yield partially fertile progeny, genetic analysis of interspecific differences is possible. Interspecific hybrids are intermediate to the parental species in basal thermotolerance and inducible thermotolerance and resemble D. virilis in Hsp concentrations after intense heat shock and Hsp70 protein electromorphs.