Two major species of glucose-6-phosphate dehydrogenase (EC 22.214.171.124) differing in size, pyridine nucleotide specificity, and susceptibility to inhibition by adenosine 5'-triphosphate (ATP) were detected in extracts of Pseudomonas multivorans (which has recently been shown to be synonymous with the species Pseudomonas cepacia) ATCC 17616. The large species (molecular weight ca. 230,000) was active with nicotinamide adenine dinucleotide (NAD) or nicotinamide adenine dinucleotide phosphate (NADP) and was markedly inhibited by ATP, which decreased its affinity for glucose-6-phosphate and for pyridine nucleotides. This form of the enzyme exhibited homotropic effects for glucose-6-phosphate. The small species (molecular weight ca. 96,000) was active with NADP but not with NAD, was not inhibited by ATP, and exhibited no homotropic effects for glucose-6-phosphate. Under certain conditions multiplicity of 6-phosphogluconate dehydrogenase (EC 126.96.36.199) activities was also noted. One form of the enzyme (80,000 molecular weight) was active with either NAD or NADP and was inhibited by ATP, which decreased its affinity for 6-phosphogluconate. The other form (120,000 molecular weight) was highly specific for NADP and was not susceptible to inhibition by ATP. Neither form of the enzyme exhibited homotropic effects for 6-phosphogluconate. The possible relationships between the different species of glucose-6-phosphate dehydrogenase and 6-phosphogluconate dehydrogenase are discussed.