Susceptibility of 16 056 strains isolated in 1982 and 1983 to fosfomycin (FOS) was tested with Mueller-Hinton medium and discs 50 micrograms FOS + 25 micrograms glucose-6-phosphate (G6P) (cutoff diameter 14 mm). For 3 411 strains, inhibition zone diameters were recorded. Almost all E. coli, Citrobacter, Salmonella and S. aureus were susceptible to FOS as well as 70% of Serratia, E. cloacae, P. mirabilis and 50% of Klebsiella and Enterobacter sp. Only 27% of P. aeruginosa, 20% of indole positive Proteus and 8% of Acinetobacter were sensitive to FOS. E. coli, Citrobacter and S. aureus seem to exhibit the highest susceptibilities to FOS. Sensitivity rates of E. cloacae, Serratia, P. aeruginosa, and especially Klebsiella seemed slightly underestimated by the disk diffusion method, but differentiation between susceptible and resistant strains was facilitated by rating as susceptible those strains with inhibition zone diameters between 12 and 14 mm. This method also seemed to underrate the sensitivity of susceptible Enterobacteriaceae, probably because of inadequate G6P concentrations at this point of the disk.