Competitive hybridization of ribosomal RNAs was used to estimate similarities of nucleotide sequences between species of Angiospermae and Gymnospermae. Similarities have been measured with respect to Cucumis sativus. In Angiospermae the nucleotide sequences are highly conserved except in the species of the family of Compositae, where the percentages of similarity are clearly lower. In the Gymnospermae the lowest similarity has been observed with Torreya californica. A relationship is hypothesized between conservation of rRNA nucleotide sequences and evolutionary position of the species.