We characterized the MHC class II B gene in the green-rumped parrotlet, Forpus passerinus. Three approaches were used: polymerase chain reaction amplification using primers complementary to conserved regions of exon 2, sequencing clones from a genomic library, and amplification of exon 2 using species-specific primers. All three methods indicate that there is only a single class II B locus in this species and no pseudogenes. We suggest that this is the ancestral state for birds. The gene is highly polymorphic; 33 alleles were found in a sample of 25 individuals. Variation in exon 2 is concentrated in the peptide binding residues which show a significant excess of non-synonymous substitutions consistent with the operation of selection in maintaining this extraordinary polymorphism. Genomic clones show that major histocompatibility complex (MHC) gene organization is different from that of chickens; the class II A locus is close to II B. These data provide support for the hypothesis that the bird MHC constitutes a "minimal essential MHC" for responding to infectious disease.