Seven-day-old chickens wee intratracheally inoculated with Mycoplasma gallisepticum. The tracheas collected 6 and 14 days after chickens were inoculated were subjected to titration of mycoplasma and examination by light and electron microscopy. The mycoplasma organisms grew well; 10(7) to 10(8) color-changing units in a milligram of tissue were determined. Tracheal lesions occurred in close association with the presence of mycoplasmas and were characterized by degeneration of the epithelial cells and inflammatory cellular infiltration of the mucosa. Mycoplasmas were predominantly found extracellularly and only rarely in phagocytic vacuoles of the epithelial cells. Although the mycoplasmas exhibited considerable pleomorphism in size and shape, most of them were oval or round, and the largest diameters were between 300 and 700 mn. Elongated and irregular forms were also observed, particularly in those mycoplasmas adhering to the epithelial cells. The organism had a limiting unit membrane, the fibrillar nuclear area, the peripheral cytoplasmic area containing numerous ribosomes, and a terminal bleb structure. Mycoplasmas attached to the epithelial cells by their blebs close to the host cell membrane. At the attachment site, neither fusion of the membranes of the mycoplasma and host cell nor injury to the host cell membrane could be demonstrated. Nevertheless, seemingly, the intimate association between the adhering mycoplasmas and the epithelial cells might be an important factor in pathogenesis of the disease.