Sixteen D elements were characterized from the chicken genome, 15 of which are extremely homologous. Early expression of this D repertoire was studied for both DJ and VDJ alleles. No N diversification occurs at either DJ or VD junctions. Only P additions were observed, the length of which does not appear restricted to a dinucleotide. A selection for the almost exclusive usage of the first reading frame of the D elements takes place during B cell expansion in the bursa, in parallel with the selection of productive rearrangements. All three reading frames were observed for the DJ allele at each developmental stage, although some bias for the first reading frame occurs already at the junctional stage. The high incidence of D-D junctions observed (25% among DJ sequences) might represent the major functional contribution of this multigene cluster in a system in which diversity will be generated later on by successive superimposed gene conversions. Other possible functions are discussed. The onset of D diversification through gene conversion between day 15 and day 18 of embryonic development is further documented.