An unidentified virus responsible for an arthritic condition in chickens (24, 25) was studied by electron microscopy. It was characterized in chorioallantoic membrane (CAM) lesions of embryonating chicken eggs and in tissue culture as to (a) particle size; (b) structure; (c) mode of replication in the cell; and (d) nucleic acid type. The mature virion is an icosahedral particle approximately 75 nm in diameter. The inner core is 45 nm, the outer coat 15 nm, and the virion has no envelope. Virus forms as crystals in the cytoplasm in association with a mass of fibrils 50–80 Å in diameter. Gel diffusion tests of an avian reovirus, the Crawley agent, against the unidentified virus show the viruses to have similar antigenicities. The form and behavior of the avian arthritis virus suggest it to be a reovirus.