Due to the importance of an undisturbed conventional morphology of diphteria bacilli for the rapid tentative diagnostic of the infection, a comparative morphologic study of 10 strains of Corynebacterium was done by cultivating them in 16 modifications of four routine media for isolation of diphtheria bacilli. The four basic media corresponded to Loeffler's and Pai's media, human serum-agar and chocolate-agar (from human blood), to which approximately 7.0 per cent glycerol, 0.04 per cent potassium tellurite, or both glycerol and tellurite, were added. A simple method for adding potassium tellurite to the coagulated serum (Loeffler) and egg (Pai) media was developed. From the results obtained Pai's medium seemed most adequate to maintain the typical conventional morphology of diphteria bacilli. Notwithstanding some strain variations, a large enhancement of production of metachromatic granules was shown addition of approximately 7 pepcent glycerol in aU media, with no substantial interference of potassium tellurite in the best suited media in this regard.