A fosfomycin disk (200 micrograms) placed on blood agar was used to select diphtheroids from clinical specimens. All Corynebacterium type strains tested, representing ten different groups of diphtheroids, and 150 toxigenic strains of Corynebacterium diphtheriae from different outbreaks, overgrew the fosfomycin disk, while other aerobic gram-positive rods had a large inhibition zone. The method increased the rate of isolation of diphtheroids in 1267 clinical specimens fivefold, from 1.4% to 7.6%, especially when the diphtheroids were found concomitantly with other bacteria. Overgrowth of the disk by other microorganisms occurred in 11% of the samples tested, but such microorganisms could easily be distinguished by culture morphology or Gram stain. Fosfomycin disks provide a simple, highly sensitive but not very specific aid for isolation of Corynebacterium diphtheriae and diphtheroids from clinical specimens.