The differing susceptibilities of ferret nasal mucosa and oesophagus to infection with influenza virus have been investigated. Evidence, obtained using organ cultures, suggested that virus was adsorbed with equal efficiency to both tissues. Electron microscopy showed virus directly adsorbed to the cilia and cell surfaces of the respiratory tissue. However, in contrast to nasal mucosa, virus adsorbed to oesophagus appeared to be mainly associated with an amorphous substance covering the epithelial cells and rarely in direct contact with cell membranes. Subsequently, virus penetrated the cells of the nasal mucosa and newly synthesized virus was recovered within 8 hr of infection. Virus apparently failed to penetrate the oesophagus and infective virus recovered probably merely represented residual inoculum.