Seven different bacteriophages were isolated from sewage with Escherichia coli K1 strains as host bacteria. These phages showed specific degrading activity for the host capsular polysaccharide, alpha 2,8-linked polysialic acid. The bacteriophage-associated endo-N-acetylneuraminidases degraded only a homopolymer of alpha 2,8-linked sialic acid and were not able to degrade other sialylated carbohydrates such as sialyl lactose. With respect to the minimum substrate size, each enzyme had its own specificity, Among the seven phages isolated, three enzymes exhibited novel endo-N-acetylneuraminidase activity in that they produced sialyl dimer as a main product and two of them degraded sialyl trimer or tetramer.