Organ cultures of bovine tracheal epithelium were infected with a rhinovirus or a strain of parainfluenza type 3 virus, and the epithelial surfaces were studied by scanning electron microscopy. When washed free from mucus, normal control cultures showed a thick carpet of normal cilia, whereas the two viruses each produced specific morphological abnormalities. In rhinovirus-infected cultures, degenerating ciliated and nonciliated cells with finely granular surfaces were rapidly extruded from the epithelium. The denuded epithelial surface was relatively smooth, and showed some evidence of squamous metaplasia. By contrast, in cultures infected with parainfluenza type 3 virus, damage developed more slowly and the epithelial surface was ultimately covered with a profuse array of short microvillous projections. In thin sections, some of these were shown to be the sites of viral maturation.