Two physically and genetically distinct forms of succinic-semialdehyde dehydrogenase have been identified in Escherichia coli B. The two enzymes could be separated by filtration on Sephadex G-150 and their apparent molecular weights were 200 000 and 97 000. The larger enzyme, which is specific for NADP, is induced by growth on gamma-aminobutyrate. Its induction is highly coordinated with that of gamma-aminobutyrate:2-oxoglutarate transaminase, the enzyme which initiates degradation of gamma-aminobutyrate. The smaller enzyme, which is induced by growth on p-hydroxyphenylacetate, has been purified to 98% homogeneity by affinity chromatography in conjunction with conventional methods. Under standard assay conditions this enzyme acts preferentially with NAD but reduces NADP at 15% of the rate observed for NAD, primarily because of a difference in Km. Apparent Km values for succinic semialdehyde and NAD are 13.3 +/- 1.3 microM and 33.7 +/- 1.4 microM, respectively. The subunit molecular weight was estimated to be 55 000, indicating that the native enzyme is dimeric. The NAD-dependent succinic-semialdehyde dehydrogenase is also induced by exposure of cells to exogenous succinic semialdehyde, a treatment which has no effect on the amount of other enzymes of p-hydroxyphenylacetate or gamma-aminobutyrate metabolism. Apparently the gene for this enzyme functions independently from the genes encoding the other enzymes of p-hydroxyphenyl-acetate degradation. As a consequence of its induction mechanism, this NAD-dependent dehydrogenase is also present in extracts of E. coli B grown with gamma-aminobutyrate as sole nitrogen source, in addition to the NADP-specific enzyme involved in gamma-aminobutyrate metabolism. Presumably the NAD-dependent enzyme is gratuitously induced by succinic semialdehyde formed by transamination of gamma-aminobutyrate.