Mycoplasma gallisepticum (MG) was used to infect chicken embryos, and scanning electron microscopy was used to examine the morphologic changes in the tracheae. Tracheae harvested from embryos infected with MG for 5 days showed extensive deciliation, surface erosion, and inflammatory cell infiltration. Embryonic tracheal explants infected with MG for 6 hr showed the same deciliation and surface erosion. The damage to the tracheal surface caused by MG at the embryonic stage might play a role in the pathogenesis of MG infection.