Euglena gracilis z contained two succinic semialdehyde dehydrogenases (EC 1.2.1.16), one requiring NAD and the other NADP, and these isozymes were separated from each other and partially purified. The NAD-linked isozyme was relatively stable on storage at 5 degrees C whereas the NADP-linked one was extremely unstable unless 30% glycerol or ethyleneglycol was added. The optimum pH was 8.7 and optimum temperature 35-45 degrees C for both isozymes. They were inhibited by Zn2+ and activated, particularly the NAD-linked enzyme, by K+. Sulfhydryl reagents activated both isozymes. The Km values for succinic semialdehyde were 1.66 - 10(-4) M with the NAD-linked isozyme and 1.06 - 10(-3) M with the NADP-linked one. The NADP-linked isozyme was induced by glutamate while the NAD-linked one was not. Probable roles of these isozymes in the physiology of Euglena gracilis are discussed.