The recombinant DNA vector, lambda Charon 4A, was used to construct a library of DNA sequences from the genomic DNA of soybean (Glycine max). To define the organization of ribosomal DNA (rDNA) in the soybean genome, clones containing sequences complementary to both 17S and 25S rRNA have been isolated from this library and used in conjunction with Southern blot hybridization. The rRNA genes are tandemly reiterated with a relatively small unit repeat length of 7.8 kb. There is no heterogeneity in the length of the rDNA repeat units although they display limited differences in either base sequence or pattern of methylation. The cloned rDNA sequences are shown to comprise the entire repeat unit and have been used to obtain a detailed restriction map as well as an approximate transcription map of soybean rRNA genes. The cloning of rDNA from soybean suggests that recombinant DNA techniques can be successfully applied to the genomic DNA of higher plants despite the high degree of methylation exhibited by plant DNA.