Abstract   The expression of two temperature-sensitive reporter genes, hsp70 and an hsp70-LacZ fusion, in free-ranging adult Drosophila melanogaster indicates that natural thermal stress experienced by such small and mobile insects may be either infrequent or not severe. Levels of the heat-shock protein Hsp70, the major inducible Hsp of Drosophila, were similar in most wild Droso- phila captured after warm days to levels previously reported for unstressed flies in the laboratory. In a transgenic strain transformed with an hsp70-LacZ fusion (i.e., the structural gene encoding bacterial β-galactosidase under control of a heat shock promoter), exposure to temperatures ≥32°C in the laboratory typically resulted in β-galactosidase activities exceeding 140 mOD₄₅₀ h^–1µg^–1 soluble protein. Flies caged in sun frequently had β-galactosidase activities in excess of this level, whereas flies caged in shade and flies released and recaptured on cool days did not. Most flies (>80%) released on warm, sunny days had low β-galactosidase activities upon recapture. Although the balance of recaptured flies had elevated β-galactosidase activities on these days, their β-galactosidase activities were <50% of levels for flies caged in direct sunlight or exposed to laboratory heat shock. These data suggest that even on warm days most flies may avoid thermal stress, presumably through microhabitat selection, but that a minority of adult D. melanogaster undergo mild thermal stress in nature. Both temperature-sensitive reporter genes, however, are limited in their ability to infer thermal stress and demonstrate its absence.