Although the most common bacteria in the supragingival plaque are Gram-positive streptococci, no extensive investigations have been conducted into the susceptibility of these species to chlorhexidine and cetylpyridinium chloride. Therefore, in this study, we investigated the susceptibility of 80 streptococcal strains in planktonic or biofilm states to these two antimicrobial agents. The minimum inhibitory concentration (MIC) and minimum bactericidal concentration (MBC) of the planktonic streptococci were measured using the microdilution method, as were the minimum biofilm inhibitory concentration (MBIC) and minimum biofilm eradication concentration (MBEC) measured on streptococcal biofilms formed on 96-well plates. In all species, the MIC, MBC, MBIC, and MBEC values were higher for chlorhexidine than for cetylpyridinium chloride, with sensitivity values varying according to species. For chlorhexidine, the MIC, MBC, and MBIC values showed statistically significant differences among species. However, only MBEC values showed statistically significant differences for cetylpyridinium chloride. The MIC against Streptococcus mutans and the MBC against Streptococcus salivarius were significantly lower than those against the other species. With he exception of a few species, most of the bacterium susceptibility values were higher in the biofilm state than in the planktonic state.