The sensitivity of 116 strains of Bacteroides fragilis and 60 strains of Sphaerophorus to fosfomycin was studied. Of the 178 strains that were tested, 156 were isolated from pathological products in our department and the remainder came from the following collections: NCTC, ATCC and the Institut für Mikrobiologie of the University of Bonn. Identification was made morphologically and biochemically in accordance with the scheme which we usually employ and which has previously been published. The sensitivities were determined by the progressive dilution method in a solid medium with a surface inoculation of the strain. The results obtained show that no matter what kind of B. fragilis was tested, none of them were sensitive to concentrations of 200 μg/ml fosfomycin. On the other hand, all strains of Sphaerophorus were sensitive to lesser concentrations than the aforementioned. The majority of these concentrations subsequently lie within therapeutic margins. The conclusion drawn from our results is that the sensitivity to fosfomycin constitutes a test for differentiating Bacteroides from Sphaerophorus and that fosfomycin is ineffective in infections caused by Bacteroides, although it is an active antibiotic against infections caused by Sphaerophorus