The selection scheme employed a two—vial apparatus in which beetles (Tribolium castaneum (CS) and T. confusum (CF)) could move in one direction between vials. One inactive line and one active line were selected for five generations in both species. A second active line in which selection for activity was more intense was selected for three generations in both species. In CS, both active lines differed significantly from the controls. The inactive line approached a significant difference. In CF, only the second active line attained a significant difference from control level. The responses developed by this selection procedure apparently were not due to reactivity to conditioned medium.