Evaluation of the inhibitory effect of a series of secondary plant compounds including steroidal alkaloids and glycoalkaloids on larvae of the red flour beetle, Tribolium castaneum, was investigated. Larval growth was inhibited on artificial diets containing 1 mumol g-1 diet of the glycoalkaloids solamargine, solasonine and tomatine, whereas the corresponding aglycones solasodine and tomatidine, and also tomatidenol, were inactive. The inhibitory effect of solamargine and tomatine, but not of solasonine, was completely abolished by addition of 1 mumol g-1 diet cholesterol and/or sitosterol. Nonetheless, synthetic cholesteryl tomatide displayed significant activity at 2 mumol g-1 diet. Parallel studies with the tobacco hornworm, Manduca sexta, showed marked inhibitory activity of tomatine at a dietary concentration of 1 mumol g-1, whereas the other compounds did not affect sterol metabolism or larval development. An appraisal of the factors influencing the mode of action of the active steroidal glycoalkaloids is attempted.