AbstractThe effect of temperature on spontaneous mutation frequencies was studied in the Drosophila somatic mutation and recombination test. Transdihybrid mwh +/+ flr3 larvae were grown at various temperatures (15, 18, 20, 22, 25, 27, and 29°C) in the absence of any chemical mutagen. Wings of surviving adults were removed and scored for presence of small single spots, large single spots, and twin spots. No significant effect of temperature was found on mean frequency per wing of twin spots. Similarly, no significant effect of temperature was found for large singles within the temperature range of 18–29°C; however, at 15èC, the mean frequency per wing of large spots was significantly elevated. A significant quadratic relationship was found between mean frequency of small spots and temperature. Frequency of small spots per wing was minimized within the temperature range of 20‐27°C and increased at higher and lower temperatures. Maximum frequency of small spots per wing was observed at 15°C. The qualitative nature of the small single spots induced at high and low temperatures is unclear; they may represent slow‐growing segmentally aneuploid (deleted) cells or possibly even monosomic cells. Both heat and cold were found to be mutagenic in the Drosophila wing‐spot assay. However, the mutagenic potency associated with temperature was much less than that of most chemical mutagens, and no significant effect of temperature was observed in the range of 20–27°C. © 1993 Wiley‐Liss, Inc.