The in vitro activity of trospectomycin sulfate was compared with those of several antimicrobials, against 301 anaerobic bacteria and 613 aerobic Gram-positive cocci. Trospectomycin was about 4- to 32-fold more active than was spectinomycin. Trospectomycin exhibited consistently good activity against all Bacteroides fragilis group isolates, except Bacteroides vulgatus, and against all other anaerobes comparable or higher to that of clindamycin. The trospectomycin's activity was most similar to that of vancomycin, even against methicillin-resistant Staphylococcus aureus and methicillin-resistant Staphylococcus epidermidis.