The wild-type strain of Rhodopseudomonas spheroides 2.4. 1 does not utilize exogenously supplied gluconate and grows poorly with glucose, fructose or mannose as substrates, accumulating aldonic acids and 2-keto-3-deoxy gluconic acid in media containing these sugars. Mutants which are capable of growing well with glucose without the accumulation of acids acquire the enzyme "phosphogluconic acid dehydrase" and oxidize hexoses mainly via the pathway described previously in Pseudomonas saccharo-phila. The Embden-Meyerhof pathway is constitute in the wild-type and mutant strains of R. spheroides but appears to have a limited function as a result of very low aldolase activity. Phosphogluconic acid dehydrogenase is either absent or too low in activity to permit the utilization of the pentose-phosphate oxidative pathway. A new type of particulate "aldose dehydrogenase system", constitutive in all strains is involved in the oxidation of glucose and mannose to the corresponding aldonic acid lactones. Gluconic and mannonic acids are both converted to 2-keto-3-deoxy acid through the action of two distinct dehydrating enzymes. A mutation leading to the utilization of gluconic acid is associated with the acquisition of a high level of "2-keto-3-deoxy gluconic acid kinase", but not of phosphogluconic acid kinase. A permeability factor may also be involved in the adaptation of gluconate.