Two anionic soluble monomers, mono(2-methacryloyl oxyethyl)acid phosphate and vinylsulfonic acid sodium salt, were grafted onto chitosan to obtain copolymers with zwitterionic property. Graft reaction improved the antimicrobial activities of chitosan. Antimicrobial activities of chitosan and graft copolymers depended largely on the amount and type of grafted chains as well as changes of pH, against Candida albicans, Trichophyton rubrum, and Trichophyton violaceum. The most excellent antimicrobial activity among tested samples was shown at pH 5.75 with demonstrating strain selectivity against Candida albicans and Trichophyton violaceum due to the difference in affinity between cell wall of fungi and chitosan or its derivatives.