An exopolysaccharide depolymerase of Rhizobium trifolii is induced in the host bacterial cells by virulent phage infection. The action of this depolymerase appeared only when exopolysaccharide contained glucuronic acid in the exopolysaccharide chains. The reaction was completely inhibited by ethylenediaminetetraacetic acid (EDTA) and this inhibitory effect was abrogated by Ca2+. This enzyme has a molecular weight of about 4.4×105. Optimum pH, optimum temperature, and minimum dose for the activity of this enzyme were elucidated. No difference in the elution pattern on column chromatography and molar ratio of component materials was found between oligosaccharides of infectious and non-infectious bacterial strains obtained after hydrolysis by the depolymerase.