The alginate depolymerase associated with bacteriophage infection of A. vinelandii was used in the analysis of sodium alginate. The enzyme degraded the polysaccharide to a series of oligouronides each containing a terminal 4-deoxy-.alpha.-L-erythro-hex-4-enopyranuronosyl residue. Analysis of these oligouronides together with kinetic information indicated that the enzyme was specific for mannuronic acid-containing regions of the polyuronide. The specificity of the enzyme made it possible to determine the primary structure of the macromolecule. The phage-induced enzyme was distinct from the alginate lyase elaborated by the host organisms by its pH optimum, molecular weight, Km and stability.