SUMMARY Polyphenism is a key strategy used by solitary insects to adapt to changing environmental conditions and by eusocial insects for existing collaboratively in a social environment. In social insects, the morphogenetic juvenile hormone(JH) is often involved in directing the differentiation of polyphenic behavioral castes. The present study examines the effects of JH, environment and feeding on caste polyphenism in a eusocial insect, the termite Reticulitermes flavipes (Kollar). Our approach included a combination of model JH bioassays, SDS-PAGE and western blotting. Our findings revealed significant temperature-dependent effects on (1) JH-induced soldier caste differentiation, (2) abundance of soldier-inhibitory hexamerin proteins and(3) JH-sequestration by hexamerin proteins. Additionally, although it appears to be dependent on a complex interaction of factors, feeding apparently plays a significant upstream role in enhancing hexamerin accumulation under normal colony conditions. These findings offer important new information on termite eusocial polyphenism by providing the first mechanistic evidence linking an intrinsic caste regulatory factor (hexamerin proteins) to an upstream extrinsic factor (environment) and a downstream response (caste differentiation). These observations are consistent with the hypothesis that the hexamerins serve as an environmentally and nutritionally responsive switching mechanism that regulates termite caste polyphenism.