A meridic diet was supplemented with avidin at various concentrations to determine its effects on growth and mortality of three lepidopteran insects: Helicoverpa zea (Boddie), Spodoptera exigua (Hübner), and Anticarsia gemmatalis (Hübner). All insects were placed on diet immediately after hatching and observed until death or pupation occurred. At a concentration of 10 ppm, avidin had little or no effect on growth and mortality compared with the control. However at a concentration of 100 ppm almost all tested insects were killed. H. zea was further tested by adding sublethal concentrations of Bt (Cry1Ac) in the diet containing avidin. The synergistic effect was significant, with mortality increasing to 44.4% over additive mortality (21.6%) of Bt and avidin.