Avian IgY is closely related to an ancestor of both mammalian IgG and IgE and thus provides insights into the evolution of antibody structure and function. A recombinant fragment of IgY-Fc consisting of a dimer of the Cupsilon3 and Cupsilon4 domains, Fcupsilon3-4, was expressed and crystallized and its X-ray structure determined to 1.75 A resolution. Fcupsilon3-4 is the only nonmammalian Fc fragment structure determined to date and provides the first structural evidence for an ancient origin of antibody architecture. The Fcupsilon3-4 structure reveals features common to both IgE-Fc and IgG-Fc, and the implications for IgY binding to its receptor are discussed.