The effects of larval phase polyphenism on adult reproductive biology, and larval nutritional and thermal ecology of the Af rican armyworm moth, Spodoptera exempta (Walker) were investigated. Differences between moths which were reared from solitaria and gregaria larvae were obtained in fecundity (weight-adjusted) and pre-oviposition period. Number of egg batches, oviposition period and longevity of female moths were inf luenced by adult di ' et and not by larval phase. Phase effects, on fecundity and pre-oviposition period were removed by rearing larvae as solitaria for three generations. Larval food consumption and utilization indices were affected by phase and instar; the direction of differences depended on whether solitaria larvae were first- or thirdgeneration. There were no differences in growth rates between phases. Weight-adjusted triglyceride contents of larvae were higher in the gregaria in two out of three trials. Solitaria larvae contained more body, -water. Rearing of larvae in different sizes of containers did not affect the triglyceride levels within phases, but affected water content in the solitaria. At constant ambient temperatures of 17.5,22.5 and 25"C, gregaria larvae developed faster than solitaria. At 300C, larval period was shorter for the solitaria than the gregaria. The rate of development did not differ between phases at 35"C. Gregaria larvae lost their black pigment at 350C and above. Anaesthetized black larvae attained higher rates of increase in temperature than larvae of lighter pigment when exposed to radiant energy. Loss of the black pigment at higher temperatures was accompanied by loss of ability of gregaria larvae to heat up faster under radiant energy. Studies on behaviour failed to demonstrate any preference of larvae in one phase for illuminated or shaded areas. The implications of these findings for the life history of S. exempta and the problems involved in research on phase polyphenism are discussed.