We selected 100 strains of organisms that were isolated in our hospital, which came from pathological products and which had an MIC of fosfomycin of 256 μg/ml or more and belonged to the genera Klebsiella, Pseudomonas, Escheríchia coli, Serratia, Enterobacter, Proteus, Acinetobacter Levinea and Staphylococcus. We have studied the effect of the association of fosfomycin with 15 antibiotics (β-lactamins, cephalosporins, aminoglycosides, etc.) and 5 chemotherapeutics. The 11 selected strains were studied by the ‘chess board’ technique in agar. A study was also carried out on the action of fosfomycin on E. coli K12 E711 in the phase of logarithmic growth. The 11 selected strains were studied bz the ‘chess board’ technique in agar. Synergic effect with fosfomycin associated to other antimicrobials was found in