The secondary structure of glycerol-3-phosphate dehydrogenase was predicted from its amino acid sequence. The pattern of helices and sheets within the first half of the polypeptide as well as specific marker residues were consistent with the properties of the NAD binding domain in other dehydrogenases. The second half of the sequence shows similarities with the catalytic domain of glyceraldehyde-3-phosphate dehydrogenase. The resulting two-domain structure of glycerol-3-phosphate dehydrogenase allows the correct environment for the B specificity of the nicotinamide ring and the L-glycerol 3-phosphate substrate.