The manA gene encoding an extracellular beta-1,4-mannanase of a marine bacterium, Vibrio sp. strain MA-138, was cloned and sequenced. The manA gene consists of an open reading frame of 1185 nucleotides encoding a protein of 395 amino acids with a molecular weight of 43,097. A putative ribosome-binding site and a promoter region were identified in the DNA sequence. ManA of Vibrio sp. strain MA-138 was classified into family 5 of the glycosyl hydrolases and is highly homologous to the ManAs of Caldocellum saccharolyticum (sequence identity : 53%) and Streptomyces lividans (sequence identity : 51%). The N-terminal amino acid sequence of the recombinant enzyme was completely in agreement with that deduced from the nucleotide sequence and that of the enzyme purified from strain MA-138. Although the molecular weight of the recombinant ManA was smaller than that of the enzyme from strain MA-138 on SDS-polyacrylamide gel electrophoresis, this was attributed to linkage of the carbohydrate chain to the latter protein. The enzymatic properties of the recombinant ManA was similar to those of the enzyme from strain MA-138.