The proportion of the genome that is complementary to 18 and 28 s has been determined for 12 species of amphibians. The group of species chosen for the experiment includes 5 related species belonging to the same genus (Plethodon) as well as species belonging to distant taxonomic groups whose C-values range from 3 to 62 pg. Hybridization of rRNA (18 S + 28 S) to whole chromosomal DNA to saturation level indicates that the proportion of rDNA decreases with increasing DNA content. The results presented in this paper do not support the hypothesis of Nardelli, Amaldi & Lava-Sanchez that the number of ribosomal cistrons in various amphibians can always be expressed as a power of 2.