25-Azacholesterol and 25-azacoprostane significantly increased the larval period of southwestern corn borer in a dose dependent fashion from 1 to 300 ppm in the diet. Male larvae were much more resistant than female larvae to the growth-inhibiting effects of the azasteroids. Both azasteroids caused melanization of larvae and inhibited moulting, but did not produce larval-pupal intermediates. The azasteroids caused a reduction in pupal and adult weight at higher concentrations, more so in females than in males. Pupal periods and adult longevity were shortened, and adult wing malformation occurred with the highest azasteroid concentrations. Egg production and hatchability decreased with increasing azasteroid concentrations in larval diets.