Recombinant Escherichia coli JM109 cells expressing β-galactosidase fused with X open reading frame of hepatitis B virus exhibited temperature sensitive production of β-galactosidase. Decrease in activity at high culture temperature (37℃) was not due to the decrease in fusion polypeptide production per se, but rather due to aggregation of the fusion polypeptides within the cells. However once the fusion polypeptides folded into proper conformation at low culture temperature (17℃), they maintained the activity even at high temperature (42℃). Aggregates of the fusion polypeptides did not convert to active form when the culture temperature was lowered. The results indicate that aggregation is not a consequence of thermal denaturation of native structure but of incorrect folding and assembly of the polypeptides at high culture temperature.