The aerobic and anaerobic microbiologic characteristics of pyomyositis were investigated through a retrospective review of microbiologic and clinical data at the U.S. Naval Hospital in Bethesda, Maryland. Microorganisms were isolated from eight abscesses. Only anaerobic organisms were recovered in three patients (37.5%), aerobic and facultatives only in three (37.5%), and mixed aerobic and anaerobic bacteria in two (25%). A total of 15 isolates were recovered (10 anaerobic, five aerobic or facultative). The isolates included three Peptostreptococcus species, three Staphylococcus aureus, two Bacteroides fragilis group, two Clostridium species, and one each of Escherichia coli, Streptococcus pyogenes, and Fusobacterium nucleatum. Predisposing conditions included trauma (three cases), malignancy (three), diabetes (one), and aplastic anemia (one).