Fosfomycin showed lower MIC values under anaerobic culture conditions than those under aerobic conditions for four Gram-positive and 18 Gram-negative bacterial isolates. The degree of change in MICs was dependent on the culture media and strains tested. The growth-inhibitory diameter in the paper disc assay increased in parallel with the decrease in the redox potential of the agar medium. Bacteriolytic activity was also potentiated in anaerobiosis. The increase of the bio-activity of fosfomycin in anaerobic culture was neither due to the change of medium pH nor to the change of mobility of drug in agar. However it is possible that the uptake of fosfomycin through the cell membrane increased in anaerobiosis. The anaerobic MICs of fosfomycin were better correlated than the aerobic MICs with the ED50 values in a mouse systemic infection model.