The ribosomal DNA (rDNA) of Cucurbita pepo L. has been found to consist of tandemly arrayed repeat units, most of which are 10 kilobases in length. Thirty-six repeat units, cloned into the HindIII site of pACYC 177, fall into seven classes which differ from each other in length and/or nucleotide sequence. Most of the heterogeneity occurs in noncoding portions of the repeat unit although there is some nucleotide sequence variation in the coding portion as well. Heterogeneity of base modification was observed in genomic rDNA of which two examples are: (a) all of the repeat units have three BamHI sites, one of which is unavailable for restriction in about half of the units and (b) all of the CCGG sites except one are methylated at the internal cytidine in many of the units; a second site is unmethylated in some of the units and in a very few units a third site remains unmethylated.