1. In the majority of tissues in the cecropia silkworm, cytochromes b and c are apparently absent, being replaced by a hitherto undescribed component which we have tentatively termed cytochrome x.
2. Spectroscopically, the new cytochrome is characterized in the reduced state by a broad and indivisible absorption band extending from 551 to 562 mµ.
3. The enzyme can be demonstrated only in the larval stage of the insect and undergoes breakdown prior to pupation. It occurs in highest concentration in the walls of the larval midgut and resists extraction, being apparently bound to subcellular particles.
4. Evidence is presented, based on spectroscopic and manometric studies in the presence of various substrates and inhibitors, that cytochrome x is a single component which mimics certain of the properties of cytochromes b and c and of succinic dehydrogenase.
5. Detailed studies served to differentiate cytochrome x from the cytochromes b1 and b2 which it resembles spectroscopically.
6. On the basis of spectroscopic studies at the temperature of liquid air, it is concluded that cytochrome x closely resembles, and may be identical with, the component which Keilin has recently described as cytochrome e.
