When fed to larvae of tobacco hornworm, Manduca sexta (L.), cyromazine strongly inhibited normal growth and development. Depending on the dose or period of feeding, symptoms were cuticular melanization, swellings in intersegmental regions, cuticular lesions, rupture of the body wall, and death. At greater than 10 ppm, cyromazine in the diet initiated symptoms in fifth instars, whereas greater than 20 ppm induced symptoms which led to death during the instar or at molting. The ratio of chitin to larval body weight in both cyromazine-exposed and control fifth instars increased slightly from ecdysis to 42 h. However, this ratio did not differ between the two groups, indicating that cyromazine had no immediate or direct effect on chitin production. When cyromazine was ingested by fifth instars, it was excreted rapidly, and a small amount (less than 5%) was metabolized to melamine. The amount of cyromazine found in the hemolymph remained relatively constant during the feeding period, whereas the amount present in the body wall increased with time and was localized in the KOH-soluble fraction. Cyromazine may inhibit growth or expansion of the body wall (or both) sufficiently to prevent normal internal growth, producing the observed symptoms and leading to abnormal development.