Cyromazine inhibits development and ecdysis of larval Lucilia cuprina (Wiedemann). This study details unique changes induced by cyromazine in the ultrastructure of the cuticle of the body, foregut, and hindgut. Apolysis and, ultimately, ecdysis are severly disrupted or inhibited in affected larvae. However procuticle continues to be secreted by the epidermal cells of body and foregut, resulting in a gross thickening of the cuticle. The cuticle of severely affected larvae is traversed by disoriented microfibrils and is also subject to abnormal sclerotization and melanization. Despite the disruption of ecdysis, new cuticle is secreted under the old one in treated larvae. In extreme cases, this results in the occlusion of the digestive tract by cuticle from two instars. This study supports previous conclusions that cyromazine does not act by inhibiting chitin formation. It is suggested that the primary mode of action of cyromazine is via the hormonal system.