SUMMARY: When examined by phase-contrast microscopy living diphtheria bacilli generally appear fairly transparent. They are sometimes unicellular but are frequently divided into compartments by readily visible cross-septa. In gravis and mitis strains the cross septa are usually few in number and the compartments long, but in intermedius strains there are often many cross-septa, and the organism may resemble a chain of cocci fused together. Club-shaped cells with swellings are seen in untreated living cultures as in fixed and stained preparations of killed bacteria, and occasionally branched forms are encountered. Undivided filaments sometime occur in magnesium-deficient cultures, whilst in magnesium-rich cultures cells with numerous cross-septa are scanty. The presence of magnesium salts has some protective effect against the growth-inhibition produced by beryllium salts. In cultures containing selenite or tellurite the bacteria become red or black, granules containing selenium and tellurium occur and areas resembling chromatinic structures may be seen.