A study was made of the effect of the inoculum size and the culture conditions on the MIC of a strain of Pseudomonas aeruginosa that was sensitive to fosfomycin. The increase of the inoculum and the conditions which favour the cellular multiplication considerably increase the MIC. Phosphates inhibit the action of the antibiotic. Fosfomycin produces a logarithmic decrease in the number surviving 1 h after being added; the bacterial lysis measured by the decrease in the OD depends on the medium and is hardly noticeable in ordinary broth. In a minimum medium the antibiotic does not affect the growth. The determination of the variation in the consumption of oxygen is a rapid and accurate method of evaluating the effect of fosfomycin. A spontaneous mutant glpT was isolated which was more than 128 times more resistant than the isogenic parental strain. The growth curves of this mutant, the sensitive parental strain and a naturally resistant strain that was isolated in a patient were similar to each other in complex media. In synthetic media the naturally resistant strain shows shorter phases of latency, but the generation times are analogous. This would seem to indicate that the resistance to fosfomycin in P. aeruginosa has no infulence on the speed of growth.