Avian infectious bronchitis virus is a ribonucleic acid-containing ether-sensitive medium-sized virus lacking direct haemagglutinating activity except under special circumstances (Cunningham, 1963; Biswal, Nazerian & Cunningham, 1966) and has been considered as a possible myxovirus on these bases (Cabasso, 1965). Recent studies of the morphogenesis of infectious bronchitis virus in chick embryos revealed its similarity to a human virus isolated from patients with respiratory disease (Becker et al. 1967) but both viruses have certain physical features which differ from those of the myxo- and paramyxoviruses (Berry et al. 1964; Nazerian & Cunningham, 1967; Becker et al. 1967). The purpose of the present study is to describe the morphogenesis of infectious bronchitis virus in cell culture. The 114th passage of infectious bronchitis virus strain 42 in chicken embryo kidney cell culture (Cunningham & Spring, 1965) was used with fibroblast cultures prepared from chicken embryos free of leucosis virus (provided by Dr R. E. Luginbuhl, Department of Animal Diseases, University of Connecticut, Storrs).