Sheared fragments of mouse satellite DNA can form rings and other circular structures by several techniques. „Folded rings“ are formed if the sheared fragments are simply annealed, indicating that shearing produces single-chain terminals, and that the repetitious sequence is shorter than the exposed ends. The occurrence of folded rings can be sharply reduced by prior treatment with single-chain specific endonuclease, and significantly increased if the fragments are treated with exonuclease III. Denaturation of satellite DNA followed by reassociation of the single chains results in the formation of “slipped rings”. These characteristics of the DNA lead to the conclusion that the sequences of the mouse satellite DNA are arranged in a tandemly repetitious manner.-About 20% of the DNA fragments from the “main band” cyclize after partial exonuclease III degradation, but not before this treatment. This indicates that a large fraction of the main band DNA is tandemly repetitious, but that the length of the repetitious sequence is on the average longer than the single-chain terminals produced by shearing.