The morphology of viable Mycoplasma fermentans, Mycoplasma hominis, Mycoplasma orale types 1 and 2, and Mycoplasma salivarium was studied in broth cultures by interference microscopy and in thin sections by electron microscopy. Only spherical cells were seen by interference microscopy. M. hominis had a capsule-like outer layer. Except for M. orale type 1, mycoplasmas in thin sections were 0.3-1 mum in diameter, with a bounding trilaminar membrane 7.5-10 nm thick. The mycoplasmas contained DNA fibrils and randomly distributed ribosomes. No polyribosomes were seen. Dividing mycoplasmas elongated slightly; the membrane invaginated, forming one bud. Sometimes M. hominis and M. salivarium produced one bud by elongation, and the bud was attached by a tube. This method of division is not considered as characteristic but rather as due to centrifugal force separating unfixed cells during preparation for electron microscopy. Cross-septa were never observed. In thin sections M. orale type 1 was elongated and without buds, an observation which suggested that preparation for electron microscopy distorted the mycoplasmas.