Rates of development were compared between lines ofTribolium castaneumselected for high and low responsiveness of egg production to conditioned medium. The selection lines were established from four strains which were each derived from another wild population in Israel. Each high-response line was compared with the low-response line of the same strain. All experiments were carried out in fresh medium. In three of the four comparisons, the high-response line proved to have a higher rate of development than the corresponding low-response line. It is suggested that the variability for responsiveness in natural populations is maintained by a balance between the selective advantage of the high responders in optimal environments and their selective disadvantage in suboptimal environments.