We have found and characterized a recombinant between the EK2 vector lambdagtWES.lambdaC and a portion of the mouse ribosomal genes. A 6.6 kb endoR.Eco RI fragment was purified from total mouse DNA using RPC-5 ion exchange chromatography and then cloned and detected twice among 183 hybrid phage screened. In situ hybridization of restriction fragments of the hybrid phage DNA revealed that the inserted fragment contained both 18S and 25S RNA sequences. Electron microscopic analysis further suggested that most, if not all, of the 28S RNA sequence was present in the insert. The orientation of the 28S sequences in the hybrid phage was such that the "sense" of the inserted fragment should be under the control of the leftward promoter of lambda.