Summary During 1 year 116 patients with paronychia had bacteriological cultures of the pus. Aerobic bacteria, predominantly Staphylococcus aureus were isolated from 81. Anaerobic bacteria were isolated from 35, and in 31 of these aerobes were also present. Anaerobic paronychias were less acute than those caused by aerobes but otherwise the clinical features and management were the same. The anaerobes isolated were those found as oro-pharyngeal commensals and did not include the colonic commensal Bacteroides fragilis. They were, with few exceptions, sensitive to penicillin.