To understand the characteristics of 29 toxigenic strains of Corynebacterium diphtheriae isolated in Seoul area, type classification, biochemical properties and antibiotic susceptibility pattern to 9 kinds of antibiotics were investigated. The results obtained were summerized as follows; I. Among the 29 strains, gravis type was the overwhelming majority(24 strains), followed by intermedius type(3 strains) and mitis type(2 strains). II. Fermentation of glucose, maltose, lactose, trehalose and mannitol, nitrate reduction and urease were tested. All strains fermented glucose, but not sucrose, lactose, mannitol and trehalose. 9 strains fermented maltose and 20 strains did not. Nitrate was reduced by 28 strains but not by one strain. In urease test one strain showed positive, 28 strains negative. III. Antibiotic susceptibility test to penicillin G, chloramphenical, kanamycin, lincomycin, streptomycin, terramycin, erythromycin and gentamycin were carried out. The MIC of erythromycin(0.025 ${mu}g/ml$ 26 strains and 0.05 ${mu}g/ml$ 3 strains) was the lowest, followed by ampicillin, lincomycin and penicillin G. Streptomycin showed the highest MIC.