Peptostreptococcus anaerobius sensu lato, currently including two closely related species, P. anaerobius and P. stomatis, is known to be more resistant than other gram-positive anaerobic cocci. We reidentified potential Peptostreptococcus isolates and tested their susceptibilities to eight antimicrobials. Notably, P. anaerobius had constantly higher values for the MIC at which 50% of the isolates are inhibited (MIC(50)) and the MIC(90) than P. stomatis.