Clostridium welchii, used as a control, was found to grow well in a microaerophilic jar used for campylobacters but appeared resistant to a metronidazole disc although the campylobacter was sensitive. Minimum inhibitory concentrations for six strains of Cl. welchii were up to 64-fold higher in these conditions than when grown anaerobically. Zones of inhibition with both Cl. welchii and Bacteroides fragilis varied with the amount of air admitted to anaerobic jars.