1. The effects of mixtures of carbon monoxide and oxygen on the growth and metamorphosis of the Cecropia silkworm were examined at successive stages of embryonic and post-embryonic development.2. Embryos, mature larvae, and adults are killed by five days of exposure to carbon monoxide/oxygen ratios of 20:1 or 25:1. Diapausing pupae, by contrast, survive at least 21 days of exposure to carbon monoxide/oxygen ratios as high as 33:1.3. While failing to interfere with the viability of diapausing pupae, carbon monoxide blocks or greatly retards the termination of the pupal diapause; it also inhibits the healing of experimental wounds in the pupal integument.4. The ability to survive in the presence of high pressures of carbon monoxide persists throughout the early stages of adult development. Exposure of the developing, post-diapausing insect to suitable pressures of carbon monoxide establishes and enforces an artificial diapause which is reversed upon return to air.5. The inhibition of adult development by ca...