The ribosomal RNA genes (ribosomal DNA) of chicken are present in approximately 200 copies and are cleaved into two fragments of molecular weight 5 X 10(6) and 12 to 14 X 10(6) by restriction endonuclease Eco RI. Recombinant phages have been constructed in vitro by joining the smaller fragment of ribosomal DNA and the outer arms of DNA from the vector lambdagtWES-lambdaC. In one of the recombinants, the coding strand of the cloned fragment is in the proper orientation for transcription with lambda "early" genes; in the other two the orientation is reversed, with the coding strand in the proper position for transcription with lambda "late" genes.