Both the proliferation and differentiation of ventral diaphragm myoblasts are controlled by ecdysteroid during metamorphosis of the moth, Manduca sexta, but the responses have different hormonal requirements. Tonic exposure to moderate levels of ecdysteroid are required to stimulate myoblast proliferation. This is due to the presence of an ecdysteroid-dependent control point in the G(2) phase of the cell cycle. As a result, proliferation can be repeatedly turned on or off simply by adjusting the concentration of ecdysteroid to be above or below a critical threshold concentration. In contrast, high levels of ecdysteroid trigger irreversible proliferative arrest and differentiation of myofibers. Myoblast proliferation and differentiation also differ in their response to the juvenile hormone mimic, methoprene. Ecdysteroid-dependent proliferative arrest and differentiation are blocked by coculture with methoprene but methoprene has no effect on ecdysteroid-dependent proliferation. In the animal, premature exposure to high levels of ecdysteroid in the absence of juvenile hormone triggers precocious differentiation of the myoblasts, resulting in the formation of several thin bands of muscle rather than a complete diaphragm. Thus, ecdysteroid and juvenile hormone collaborate to determine the size and shape of the adult musculature.