Fibroblast growth factor receptors (FGFRs) have three extracellular domains that belong to the immunoglobulin superfamily. We have determined the outline structures for these domains on the basis of their homology to the I set molecule telokin. The outline structures describe the relative positions of residues in each domain; their major secondary structures, and the extent to which residues are accessible to the solvent. They also provide the basis of a coherent description of the change in recognition properties that occur when the IIIb and IIIc exons are switched and of the effects of mutations in FGFRs that cause genetic diseases.