The polyhydroxy alcohol, mannitol, was identified in the body of the cotton aphid, Aphis gossypii Glover. Mannitol accumulated in the bodies of aphids during the course of the day, analagous to the previously reported accumulation of sorbitol in the silverleaf whitefly, Bemisia argentifolii Bellows and Perring. In both organisms, polyol synthesis was stimulated by elevated temperatures. Hemolymph polyol during mid afternoon in these insects reached approximately 500 mM. Polyol concentrations during early morning hours was approximately 10-fold less than during mid afternoon. Enzyme assays of extracts prepared from adult insects showed that fructose was the substrate for mannitol synthesis in A. gossypii. This fructose originated in the insect's diet of sucrose. The enzyme catalyzing this reaction, an NADP(H)-dependent ketose reductase/mannitol dehydrogenase, is analagous to the NADP(H)-dependent ketose reductase previously shown to produce sorbitol from fructose in B. argentifolii. We did not find evidence of a glucose to sorbitol interconversion in whiteflies, and whitefly honeydew was found to contain only traces of this polyol. Likewise, cotton aphid honeydew did not contain mannitol. In addition, glucose was not converted to mannitol by aphid extracts in the presence of either NADH or NADPH. This suggests that in both insects polyols are converted back to fructose during periods when hemolymph concentrations decrease. Whiteflies living on water-stressed plants accumulated higher concentrations of sorbitol than those on well-watered plants. Whiteflies which were allowed to feed and thereby accumulate sorbitol were more resistant to elevated temperatures (35-50 C) than those prevented from feeding. These results suggest that polyol accumulation in these insects is a physiological adaptation for their survival in hot, dry environments.