Domestic fowl were exposed to an aerosol of infectious bronchitis virus. Birds were killed at intervals after infection and sections of trachea were examined in the electron microscope. The virus replicated in superficial cells,i.e., ciliated epithelial cells and mucus cells. Both these cell types were desquamated over large areas but the underlying basal cell layer was not significantly damaged. At an early stage of replication, virus particles were confined to a small vacuolated region of cytoplasm but in desquamated cells the whole cytoplasm was vacuolated and the vacuoles contained virus particles. Virus was not found in the nucleus and all cytoplasmic particles were within vacuoles. Virus formed by budding at vacuole membranes and the vacuoles involved were derived from both Golgi vesicles and endoplasmic reticulum. The virus probably has a linear internal component and is limited by a trilaminar membrane bearing projections which tend to stain lightly. Regeneration of morphologically normal tracheal epithelium occurred between 6 and 7 days after inoculation but a second wave of virus replication in the trachea was not detected although the virus could be recovered from inoculated birds for up to 14 days following inoculation.