The effects of red kidney bean, Phaseolus vulgaris cv. Goli (L.), seed proteinaceous extract on larval development, digestive alpha-amylase and gut protein pattern of Colorado potato beetle, Leptinotarsa decemlineata (Say) were investigated. The results showed that four proteinaceous ammonium sulfate precipitated fractions (30, 50, 70, and 100%) of bean seed extract caused significant inhibition, in vitro, on alpha-amylase activity of all larval stages and adults up to 69%. Affinity chromatography using larval gut sample as ligand and silicon dioxide as matrix, were used to identify enzyme inhibitor proteins, but purified protein was not recognized on SDS-PAGE. Alpha-amylase activity of larvae fed on the treated leaves of four different potato cultivars (Agria, Burren, Picasso, and Marx) by bean seed extract was inhibited significantly up to the 60% on Burren. In the bioassay, weight of fourth instar larvae was reduced 19 mg and the larval growth was decreased by 45.35% on Picasso, as compared to the control. In two-dimensional electrophoresis and SDS-PAGE, the gut protein patterns were changed in effects of feeding on leaves treated with bean seed extract. These data revealed that red kidney bean seed extract suppressed digestive alpha-amylase activity in vitro and in vivo, caused side effects on the actual parameters monitored and had an impact on the gut protein pattern of L. decemlineata.