The region of the duck IgH locus extending from upstream of the proximal diversity (D) segment to downstream of the constant (C) gene cluster has been cloned and mapped. A sequence contig of 48,796 bp established that the organization of the genes is D-JH---. No evidence for a functional homologue (or remnant) of a gene was found. The gene is in inverted transcriptional orientation: class switch to IgA expression thus requires inversion of the approximately 27 kb region that includes both and genes. The secreted form of duck and are each encoded by 4 C region exons, and the hydrophobic C terminal regions of the membrane receptor forms of and are encoded by one and two transmembrane (TM) exons, respectively. Putative switch (S) regions were identified for duck and u by comparison with chicken S and S sequences, and for duck by comparison with mouse S. The duck IgH locus is rich in complex VNTR (variable number tandem repeats), which occupy approximately 60% of the sequenced region, and occur at a much higher frequency in the IgH locus than in other sequenced regions of the duck genome.