The structure of the paratympanic organ in chickens was investigated by means of the transmission electron microscope. The epithelium lining the lumen of the paratympanic organ consists of sensory and non-sensory components. The sensory epithelium is composed of supporting and hair cells. The hair cells are similar to the type II receptor cells present in the neuroepithelia of the vestibule and of the lateral line organs. The afferent synapses at the bases of the hair cells are also described. The non-sensory epithelium is made up of cells with a clear cytoplasm and arranged in a single layer. It also contains dark, flattened cells which sometimes possess motile cilia. Special emphasis is given to the fact that the results agree with Vitali's theory that the paratympanic organ and the lateral line organs are homologous. It is concluded, therefore, that present knowledge about this structure is not yet sufficient to allow a definitive functional interpretation.