Fifth-instar larvae of the tobacco hornworm underwent a supernumerary larval molt under several feeding regimens. Supernumerary molting occurred only when the molt was initiated at a live weight less than about 3 g; at higher weights pupation took place. The supernumerary sixth instars were normal in appearance and behavior and went on to form normal pupae only when the fifth instar had begun at weights less than 1 g. Otherwise the supernumerary sixth instars exhibited precocious metamorphosis of their crochets and imaginal discs and failed to undergo normal pupation. Thus, the commitment to pupal development appears to occur in stages. A first stage, initiated at a weight of about 1 g, commits the crochets and imaginal discs to metamorphosis at the next molt. A second stage, entered at a weight of about 3 g, permits the full and complete pupation of the remaining tissues; this transition probably reflects the elimination of juvenile hormone (JH). Starvation apparently alters the normal patterns of ecdysone production, breakdown, or sensitivity so as to elicit ecdysone-dependent development at lower than normal weights. When starvation results in fifth-instar larvae initiating a developmental response at weights below 3 g, a supernumerary larval molt occurs. But in this case the critical effect of starvation is on the timing of the molt rather than on the JH titer, since the latter even in normally fed early fifth instars is high at weights below 3 g.