MICs of fosfomycin trometamol were estimated for 40 strains of bacteria (20 gram-positive cocci, 20 gram-negative bacilli) by the agar incorporation method (Iso-Sensitest agar) in the presence of the potentiating agent, glucose-6-phosphate (25 mg/l). Titrations were carried out in duplicate under aerobic and anaerobic conditions. For 22 strains (12 gram-negative bacilli), a fourfold or greater reduction in MIC was observed in tests conducted under anaerobic conditions. The effect was particularly marked with Klebsiella spp., four of five strains of which showed a 16- to 32-fold reduction in MIC in anaerobic conditions. To investigate the reasons for the effect of anaerobiosis, selected strains were examined in an opacity monitoring device in which cultures can be grown in aerobic or anaerobic atmosphere. Surprisingly, the effect of anaerobiosis observed by continuous turbidimetric monitoring was much less than that seen in agar incorporation MIC titrations: under anaerobic conditions, there was little or no reduction in the concentration of fosfomycin trometamol required to cause a lytic effect on dense bacterial cultures, and a small, but variable effect on the emergence of resistant variants.