The presence of a specialized terminal region in Mycoplasma pneumoniae was seen in thin sections viewed in an electron microscope. Actively growing cells were examined by the freeze-fracture technique in the absence of fixation to further establish the core as a significant structural entity. Cross fractures revealed a cytoplasmic matrix surrounding a central core structure of about 54 nm. This structure disappeared rapidly in aging cells. The convex protoplastic faces of the membrane around the core had characteristic 5- to 10-nm intramembrane particles evenly distributed across the cell surface, with no apparent difference in the region of the specialized tip. A periodicity previously noted in negatively stained preparations was clearly defined here in thin sections. Attachment of actively growing cells to sheep erythrocytes was seen primarily as a side attachment rather than attachment at the tip alone. This association between the mycoplasma and the sheep erythrocytes seriously deformed the sheep erythrocytes, but no membrane fusion could be detected.