A bacterium producing the $eta$-mannosidase was isolated from intestine of fresh fish. The isolate YB-29 has been identified as Chryseomeningosepticum on the basis on its 16S rRNA sequence, morphology and biochemical proper The $eta$-mannosdiase activity was detected in both the culture filtrate and the cell extract of C. meningosepticum YB-29. The $eta$-mannosidase of culture filtrate showed the maximum activity for hydrolysis of para-nitrophenyl-$eta$-D-mannopyranoside at pH 5.0-6.0 and 55-$60^{circ}C$. The enzyme was able to hydrolyze the oligomeric substrates such as mannobiose and mannotriose with higher activity for mannotriose than mannobiose.