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Labelled chloroplast rRNAs from Spinacia oleracea were hybridized to restriction endonuclease digests of chloroplast DNA from Oenothera hookeri and Euglena gracilis, to mitochondrial DNA of Acanthamoeba castellanii, and to DNA of the E. coli rrn B operon in the transducing phage lambda rifd 18. The degree of homology is greatest for the 16S rRNA gene. Greater than 90% occurs between the two higher plant genes, 80% homology to the lower plant gene, 60%-70% homology to the bacterial gene, and 20% homology to the mitochondrial gene. The degree of hybridization varied considerably for the 23S and the 5S rRNA genes. Very high homology exists between the two higher plant genes, only about 50% homology for both the Euglena and bacterial genes, and no significant homology for the mitochondrial genes. These results show that any chloroplast (or E. coli) rRNA may be used as a probe to identify rRNA genes in other ctDNAs. Two RNA populations, each enriched for a different ctDNA-encoded mRNA, proved useful in the location of these genes on both higher plant ctDNAs. No significant hybridization was obtained using these probes to the Euglena ctDNA which seems to be too distantly related.