The extent of methylation of the 18S–25S ribosomal RNA genes ofPetunia hybrida has been investigated using methylation-sensitive restriction endonucleases and Southern hybridisation. The genes in leaves, ovaries, petals, stigmas, anthers, and seed are methylated to a similar extent, but those in roots are hypomethylated. Genes in adventitious roots formed from stem cuttings are also hypomethylated. Genes in callus cultures initiated from leaf discs vary widely in the extent of methylation, but the pattern obtained is stable during culture. Callus cells containing hypomethylated genes can give rise to shoots in which the extent of methylation has reverted to that of the original leaf explant. These results demonstrate the striking plasticity of ribosomal RNA gene methylation inPetunia.