Tween purple agar containing 1% fructose (TFP agar) differentiatedCorynebacterium genitaliumfromC.pseudogenitalium, which respectively formed colorless and yellow colonies after 72 h incubation at 37 °C aerobically or in 5–10% CO2in air. Thus TFP agar is a differential medium. Corynebacteria-like colonies grown on nonspecific urethritis (NSU) chocolate agar from urogenital material were identified asC.genitalium,C.pseudogenitalium, or commensals when subcultured on TPF agar. TFP agar was unsuitable for their primary isolation since the commensals turned the medium yellow with 24 h incubation. Gentamicin cannot be employed as a selective agent in medium for the isolation of these corynebacteria. TFP agar containing 10 μg/mL gentamicin inhibited most strains ofC.pseudogenitaliumandC.genitaliumisolated from urogenital infections. It did not inhibit isolates of these corynebacteria from cancer patients or suppress the normal bacterial flora of the urogenital tract. Evidence that gentamicin-resistant strains are characteristic of nosocomial infections is presented.