A metabolic key enzyme malate dehydrogenase (MDH) was purified from a deep-sea psychrophilic bacterium, Vibrio sp. strain no. 5710. The enzyme displayed an optimal activity shifted toward lower temperature and a pronounced heat lability. A gene encoding this enzyme was isolated and cloned. Recombinant Escherichia coli cells harboring the isolated clone expressed MDH activity with temperature stability identical to that of the parental psychrophile. Nucleotide sequencing of the gene revealed that its primary sequence was similar to that of a mesophile E. coli MDH (78% amino acid identity), for which the three-dimensional structure is known. The enzyme is thus suitable for the analysis of molecular adaptations to low temperatures.