Addition of adenine to the culture medium results in a depression of the labelling of RNA in Candida utilis growing exponentially in the presence of radioactive glucose as the sole carbon source (Barker, Jechova, Nicholson & Thompson, 1964). Evidence suggests that the effect may be due in part to changes in the contribution of the various metabolic pathways concerned in ribose formation. The reactions involved in the formation of ribose in C. Utili8 have not been fully defined. Sowden