Using the organomercuric compound 3,6-bis(acetatomercurimethyl)dioxane in conjunction with CS2 S04 density gradient equilibrium centrifugation a (dG+dC)-rich DNA fraction constituting 10% of the ovine genome was separated from the remainder. Further fractionation allowed four distinct classes of DNA to be identified with buoyant densities in neutral CsCI of 1· 714, 1· 717, 1 ·725 and 1·716 g/cm 3 • Computerized curve resolving of the data indicated the presence of several additional DNA classes.Data obtained using restriction endonuclease digestions indicated that the 1·714 g/cm 3 satellite consists principally of an 820-base pair (bp) tandemly repeating unit. The 820-bp DNA is also present in the 1· 717 and 1· 725 g/cm 3 satellites but as a minor component. The principal components of the 1·717 and 1·725 g/cm 3 fractions are 125-, 176-and 235-bp fragments. In addition, these fractions contain 705-bp tandemly repeated material. Two or possibly three species of 22-bp tandem repeats were found only in the 1·725 g/cm 3 DNA.