Two forms of succinic-semialdehyde dehydrogenase have been isolated in Pseudomonas putida. The two enzymes could be separated by filtration on Sephacryl S-300 and their apparent molecular weights were approx. 200,000 and 100,000. The smaller enzyme, which is induced by growth on 4-hydroxyphenylacetate, has been purified to 88% homogeneity by anion-exchange and affinity chromatography. Electrophoresis in sodium dodecyl sulphate gave rise to a molecular weight of 53,000, indicating that the native enzyme is dimeric. Under standard assay conditions this enzyme acts preferentially with NAD but reduces NADP at 9% of the rate observed for NAD. The large enzyme, which is dependent on NADP, is induced by growth on putrescine and its induction is highly coordinated with putrescine: 2-oxoglutarate transaminase, gamma-amino-butyraldehyde dehydrogenase and gamma-aminobutyrate: 2-oxoglutarate transaminase activities. Activity and stability conditions and true Km values for substrate and cosubstrates of the two enzymes were determined.