The low-complexity "gene-sized" linear DNA duplexes of the Oxytricha macronucleus sport short inverted terminal repeats; thus, each single strand is capable of forming a circle held together by a duplex "neck" [Wesley, R. D. (1975) Porc. Natl. Acad. Sci. USA 72, 678--682]. We have isolated necks from total, circularized, single-stranded macronuclear DNA by treatment with nuclease S1. Necks represent at least 2.2% of the total DNA, are homogeneous in size (23 base pairs), melt at 55 degrees in 0.18 M Na+, and reassociate extremely rapidly at 22 degrees (Cot1/2 = 1.1 X 10(-5) mol-liter-1.sec) to form hybrid necks of the same thermal stability. From these and other results, we conclude that all necks on all the many thousands of different single-stranded circles are the same. The neck sequence is therefore highly repetitious--found in multiple copies (as inverted terminal repeats at flush duplex ends and probably also internally) on each natural "gene-sized" macronuclear DNA molecule--implying the possible participation of this sequence both in the general vegetative metabolism of macronuclear DNA and in the pre-vegetative process whereby macronuclear DNA is excised from the total Oxytricha genome.