This study compared the in vitro activities of ertapenem, ceftriaxone, co-amoxiclav, ciprofloxacin and piperacillin-tazobactam against 314 aerobic bacteria and of ertapenem, piperacillin-tazobactam, cefoxitin, ceftriaxone, chloramphenicol, ticarcillin-clavulanate, ampicillin-sulbactam, clindamycin and metronidazole against 500 anaerobic bacteria from 212 patients with acute pelvic infection. Antimicrobial susceptibilities were determined by broth microdilution (aerobes) or agar dilution (anaerobes), following NCCLS guidelines. The most common isolates were Enterobacteriaceae and Peptostreptococcus spp. Ertapenem was the most active drug tested against Enterobacteriaceae (100% susceptible) and anaerobes (99.8% susceptible); the least active agents were co-amoxiclav (79% of Enterobacteriaceae susceptible) and ceftriaxone (85.9% of anaerobes susceptible). All agents tested had excellent activity against beta-haemolytic streptococci and methicillin-susceptible Staphylococcus aureus.