By cross‐hybridization in low stringency conditions, using a probe derived from an HLA‐DQ beta cDNA clone, we have isolated several chicken genomic DNA clones. These clones were mapped to the major histocompatibility complex (MHC) of the chick (B complex) by virtue of their ability to detect restriction enzyme length polymorphisms between congenic lines of chicken. Evidence was obtained for the presence of at least three B‐L beta genes in the chicken genome. The B‐L beta genes are transcribed specifically in tissues containing cells of the B lymphocyte and myeloid lineages and expressing the B‐L antigens. Exons encoding the beta 1, beta 2 and transmembrane domains of a B‐L beta chain have been identified with 63, 66 and 62% similarity with the HLA‐DQ beta sequence. This first isolation of an MHC class II gene outside of the mammalian class provides insight into the evolution of MHC genes based on the comparison of avian and mammalian class II beta chain amino acid and nucleotide sequences.