Abstract 1. 1. Alginate lyases from a marine bacterium, Pseudomonas sp., were partially purified from the culture medium and from bacterial cells. 2. 2. The extracellular enzyme (mol. wt 32,000) was mostly specific for polyguluronan and the intracellular enzyme (mol. wt 94,000) was more preferential for polymannuronan than for polyguluronan. 3. 3. These enzymes showed optimal pH at around 7.5 and high stability over the pH range of 6.5–8.5. 4. 4. Both these enzymes were endo-type alginate lyases, as judged from the change inviscosity of alginate solution and the analyses of the digestion products from each homopolymer on gel filtration. 5. 5. Eminent change in circular dichroic spectrum of polyguluronan was observed on the digestion with the extracellular enzyme.