A 50 year old woman while undergoing severe treatment for rheumatoid arthritis, developed anaerobic meningitis. The cerebrospinal fluid (CSF) sample was transported and cultivated aerobically and anaerobically. After 48 h at 37 degrees C the anaerobically incubated plate, the enriched fluid thioglycollate medium and the anaerobic culture medium yielded luxuriant growth of an anaerobic Gram negative bacillum. The biochemical and antimicrobial susceptibility patterns were consistent with those for Bacteroides distasonis. Most of the strains of the 5 species included in the Bacteroides fragilis group (B. fragilis, B. vulgatus, B. ovatus, B. thetaiotaomicron and B. distasonis) are resistant to penicillins, cephalosporins of first generation and aminoglycosides. Anaerobic polyresistant flora from an intraabdominal focus (chronic cholecystitis) might have been selected by treatment with gentamicin and cephalotin, and proliferated into meningeal dissemination. It is important that CSF from immunocompromised patients with acute or chronic pulmonary, intraabdominal or cranium-facial infectious processes be transported and cultured in aerobic and anaerobic conditions. These patients must be treated with an initial therapeutic scheme that includes an effective antibiotic for the anaerobic microorganism that may be involved.