The size of Corynebacterium diphtheriae is very variable, essentially rod-shaped but often showing irregular expansion at one end. Cells frequently lie in small clusters at acute angles to each other in V, L or Y shapes. Gram positive but readily decolorized, non-motile, non-capsulate, non-sporing.

Strains are distributed as beans and stained unevenly. With Metachromatic Granules Stain, deeply stained granules known as metachromatic granules, are found on one side or both sides of diphtheriae.