Glycoalkaloids are found throughout the genera Solanum (potato) and Lycopersicon (tomato). Certain glycoalkaloids, i.e., α-tomatine, solanocardenine, and leptine, have been implicated as resistance factors to the Colorado potato beetle, Leptinotarsa decemlineata Say. The allelochemical properties of these glycoalkaloids have primarily been demonstrated by studies in planta, correlating Colorado potato beetle resistance with high levels of foliar glycoalkaloids: solanocardenine in S. neocardenasii, α-tomatine in S. pinnatisectum, and leptine in S. chacoense. Although the evidence that these glycoalkaloids mediate resistance is compelling, controlled analyses of Colorado potato beetle response to purified glycoalkaloids, fed to insects in synthetic diets, are necessary to characterize the allelochemic nature of these compounds. In this study, Colorado potato beetle reared on a meridic, synthetic diet supplemented with increasing concentrations of α-tomatine exhibit retarded growth and delayed development. These effects were evident throughout the insects' development, from egg to prepupal stage. Tomatidine (the aglycone of α-tomatine) has no effect on Colorado potato beetle, suggesting that the tetrasaccharide moiety of the glycoalkaloid is essential for insecticidal activity, consistent with a membrane-lytic mechanism of action.