Little is known about the organization of class I genes in the rat although there is prima facie evidence that it is distinct from that of the mouse. We report the cloning of 61 nonclassical rat class I genes into cosmid clusters with a total mapped length of 1,264 kb. It is certain that the total number of class I genes in the rat must exceed this number. From restriction maps it is possible to identify substantial regions of duplication. By transfection of cosmids into mouse L cells, it has been possible to demonstrate at least seven different nonclassical rat class I genes that are expressible on the cell surface. Crossreaction of a single mouse monoclonal antibody with all of these class I molecules is consistent with sequence homogenization within the rat nonclassical system. Attempts to find rat homologues of the mouse T/a genes by crosshybridization of rat cosmids with a range of different TLa-specific probes were unsuccessful, suggesting that this large group of divergent class I genes is absent or nearly so from the rat. The large number of class I genes in the rat appears to have arisen by expansion of genes more closely related to the classical sequence.