Abstract The localization, amplification, and multiplicity of the rRNA cistrons in gametes of the coot clam, Mulinia lateralis, have been investigated using isopycnic centrifugation and DNA-RNA hybridization. The rRNA cistrons appear to be localized in a high-density satellite which makes up 3–5% of the DNA in both gametes. Oocyte DNA binds, on the average, 1.6 times as much rRNA as does sperm DNA. The mitochondrial DNA component of oocytes accounts for 19% of the total DNA, as estimated by denaturation and reannealing followed by isopycnic centrifugation. Hence, the ribosomal cistrons are amplified about twofold ( 1.6 0.81 ) in these oocytes. The haploid Mulinia genome contains approximately 120 cistrons for rRNA, as measured by hybridization with Xenopus rRNA at saturating concentration. The Mulinia oocyte therefore contains many fewer copies of rDNA than does the oocyte of Spisula, a closely related species, in which the ribosomal cistrons exist in greater multiplicity (195 cistrons; Collier, J. R. (1971), Exp. Cell Res. 69, 181–184), and are amplified at least fivefold (Brown, D. D., and Dawid, I. B. (1968), Science 160, 272–280). Since the oocytes of these two clams are the same size and begin rRNA synthesis at the same time after fertilization, the magnitude of rDNA amplification must not be strictly related to the needs of the developing embryo.