Abstract We isolated a bacterial strain, OS-ALG-9, which solubilized immobilized gel with calcium alginate in a reactor. It was identified as Pseudomonas sp. We studied the cultural conditions by using a 2.5- l jar fermentor and found that the growth and alginate-degrading enzyme production were optimal at pH 7.0 and 30°C. The bacterium produced alginate-degrading enzymes both intra- and extracellularly. We attempted to purify them, and found that two kinds of alginate-degrading enzyme were contained in each intra- and extracellular fraction. We only succeeded in purifying one of these, from the intracellular fraction, to homogeneity by CM-Sephadex C50 ion exchange chromatography, Toyopearl HW-55 gel filtration, and preparative polyacrylamide gel electrophoresis. The purified enzyme B was shown an optimal reaction at pH 7.5 and 45°C, was stable upto 40°C for 1 h, and was heat-inactivated logarithmically at 55°C with a half life of 8.5 min. Its molecular weight was estimated to be 45,000. The degree of final degradation of alginate was 13%. The enzyme was concluded to be an endo-type alginate lyase.