Experiments on life history genetics are usually performed using constant temperature environments in the laboratory. However, the dynamics of insect growth can be influenced profoundly by daily fluctuations in temperature such as those which characterize field environments. We report here on experiments using different stocks and selected lines of a tropical butterfly, Bicyclus anynana, to examine whether genotype–environment interactions occur for three traits describing pre–adult growth. These traits were measured over two pairs of environments differing in mean temperature, each of which had a constant, and a cycling temperature regime. Development time, pupal weight and growth rate show genotype–environment interactions, especially at comparatively low average temperatures. Researchers should, therefore, take care when extrapolating from the form of genetic covariance matrices and ‘trade–offs’ among life history traits found in constant temperature environments to those likely to occur in nature.