1. American paper products contain an extractable, heat-stable lipid (PF) with high juvenile hormone activity for the bug, Pyrrhocoris apterus. When reared in contact with "active paper" or extracts prepared front active paper, larval Pyrrhocoris undergo one or more supernumerary larval moults and finally die without completing metamorphosis or attaining sexual maturity. 2. As a potent analogue of the juvenile hormone of Pyrrhocoris, PF blocks those aspects of development which can take only in the absence of juvenile hormone—namely, the transformation of the fifth stage larva into an adult. By contrast, it fails to interfere in any way with normal growth and moulting during the first four larval instars—i.e., with developmental events which normally take place in the presence of high levels of endogenous juvenile hormone. 3. Sensitivity to PF is maximal at the outset of the final (fifth) larval instar. At this particular stage, metamorphosis is blocked by contact with PF for a period as brief as one hour. High sensitivity persists during the first two days of the fifth instar and then declines to zero after the third day. In each region of the body, the loss of sensitivity to PF is synchronized with the activation of the epidermis, as signaled by its detachment and retraction from the old larval cuticle. 4. Several lines of evidence suggest that, after its entry into the larval insect, the paper factor is more stable, and therefore more long-lasting in its effects, than is the authentic juvenile hormone of Pyrrhocoris.