Sheep and goats possess closely similar satellite DNAs which seem to be concentrated in the heterochromatin and the nucleolus. In both species, the major and minor satellites have buoyant densities of 1.711 g·cm −3 and 1.718 g·cm −3 and represent 12% and 2.5%, respectively, of the total DNA. Major satellite concentration in the combined heterochromatin and nucleolar fraction was 19%, and in the pure nucleolar fraction it was 51%. Most hitherto reported satellite DNAs have varied markedly in density and concentration from species to species, even within the same genus. The close similarity of the sheep and goat satellite is discussed in the light of what is known about the evolutionary relationship between the sheep and the goat and in the light of various hypotheses concerning the possible functions of satellite DNA.