We examined effect of fosfomycin (FOM) on growth of Escherichia coli (E. coli) NIHJ JC-2, multidrug-resistant Pseudomonas aeruginosa (P. aeruginosa) PFS80 and Staphylococcus aureus (S. aureus) Smith. FOM inhibited the growth of these organisms at 0.1 approximately 0.5, 10 approximately 50 or 5 approximately 50 micrograms ml, respectively. In E. coli, FOM clearly lyzed the cells at 0.5 microgram/ml. These 3 bacteria incorporated radiolabeled FOM proportionately to incubation time. Intracellular concentration of FOM was estimated by considering the water content of E. coli cells. The ratio of intracellular to extracellular concentrations of FOM was larger than 1 after 5 approximately 10 min. incubation with FOM and reached about 7 after 40 min. FOM was incorporated actively into P. aeruginosa over extracellular concentration after 40-min. incubation on the basis of the above-mentioned water content. It was suggested that FOM was incorporated actively into various bacterial cells, and inhibited efficiently cell wall peptidoglycan synthesis, thus inhibited bacterial growth or lyzed the bacterial cells.