The influence of glucose-6-phosphate (G6P) on the fosfomycin minimum inhibitory concentration (MIC) of six Serratia marcescens strains was determined using two standard media (Müller-Hinton and nutrient agar) and three synthetic media. G6P did not affect fosfomycin MIC when S. marcescens was grown on standard media, but on synthetic media the presence of 5–100 mg/1 G6P lowered the MIC by 1–7 log2. Five fosfomycin-resistant mutants were grown on synthetic media. In the presence of G6P, four of five mutant strains became more sensitive (MICs were 128 mg/1 or less). The fifth mutant strain remained resistant under all culture conditions. The implications of these results are discussed with regard to the mechanism of action of fosfomycin, and the practical determination of the MIC in the cl