Capnocytophaga gingivalisis most often isolated as normal oral flora or with periodontal disease. This organism is also associated with sepsis usually in immunocompromised hosts. We identified pyogenic arthritis caused byC. gingivalisin a 3-year-old immunocompetent male, whose clinical course closely resembled monoarticular onset pauciarticular juvenile rheumatoid arthritis. This is the first report ofC. gingivalisseptic arthritis in the world literature, but there are increasing reports of infections with this carbon dioxide-loving organism at other sites in nonimmunocompromised individuals. The subacute presentation of the monoarthritis with this organism of low virulence led to a long delay in diagnosis and treatment. Any monoarthritis must continue to raise concern about infection.