The discovery of proteins that control insects is critical for the continued growth of the agricultural biotechnology industry. A highly efficacious protein that killed boll weevil (Anthonomus grandis grandis Boheman) larvae was discovered in Streptomyces culture filtrates. The protein was identified as cholesterol oxidase (E.C. 1.1.3.6). Purified cholesterol oxidase was active against boll weevil larvae at a concentration (LC50 = 20.9 micrograms/ml) comparable to the bioactivity of Bacillus thuringiensis proteins against other insect pests. Histological studies demonstrated that cholesterol oxidase lysed the boll weevil midgut epithelium, suggesting that this is the primary mechanism of lethality.