The variability among Streptococcus pneumoniae isolates of capsular group 9 was analyzed with a panel of six monoclonal antibodies to pneumococcal proteins. These antibodies differentiated the 104 group 9 isolates into 18 protein types and a group (P0) not typable with the antibody panel was used. Capsular type was determined for 87 isolates: 70% were capsular type 9V, 28% 9N, and 2% 9A. In terms of protein type, the 9V isolates were four times as diverse as the 9N isolates. Significant associations were observed between the protein type, geographic origin, and year of isolation. Statistically significant associations were also observed between different manifestations of pneumococcal disease and protein type of the capsular type 9V isolates. Thus, even within capsular types, pneumococci can be highly diverse; pneumococcal protein types may be useful in epidemiologic studies to distinguish related strains in the environment.