Benzimidazole antihelmintics, including mebendazole, have a broad antiparasitic spectrum. These drugs play a major role in the treatments of parasites of intestines or other organs of vertebrates, humans, and other animals. The impact of mebendazole on the biology of the greater wax moth, Galleria mellonella (L.), was assessed by observation of several developmental parameters as follows: survivorship, developmental time, and adult longevity. Sublethal toxicity was measured through reproductive parameters such as fecundity and hatchability. The larvae were reared on artificial diet from first-instar larvae to the adult stage in the laboratory. The diets contained mebendazole at different concentrations of 0.005, 0.05, 0.5, or 1.0%. Control diet did not contain mebendazole and produced seventh-instar larvae in 96.6 ± 1.67% of cases, whereas the addition of mebendazole into diet at 1.0% significantly decreased survivorship of seventh-instar larvae to 79.9 ± 4.08%. The diet with the highest concentration of mebendazole decreased survivorship in the adult stage from 79.9 ± 2.35 to 56.6 ± 4.73%, and shortened the developmental time for adult emergence from 36.7 ± 0.48 to 34.1 ± 0.63 d. All mebendazole concentrations shortened adult longevity and significantly decreased fecundity and hatchability of G. mellonella. The highest dietary concentration of this antihelmintic significantly decreased the egg number to 28.6 ± 2.89 and hatching rate to 51.7 ± 1.85%. The present study demonstrates that mebendazole exhibits significant adverse effects on greater wax moth, leading to deteriorated life table parameters and decreased adult fitness.