By serial subcultivation on media containing egg-white lysozyme a highly resistant variant of M. lysodeikticus bacteriophage was selected. The parent strain was sensitive to 0.1 pg per ml. lysozyme while the variant was resistant to 4000 pg per ml. The two strains were examined in detail and it was found that the resistant strain differed genotypicelly from the sensitive strain in its ability to acetylate certain cell wall hydroxyl groups. This was the sole reason for lysozyme resistance. It was found that deacetylation of the resistant strain by ample chemical treatment restored its sensitivity to lysozyme while acetylation of the sensitive strain rendered it resistant.