We describe a repetitive DNA region at the 3' end of the mitochondrial DNA (mtDNA) control region and compare it in 21 carnivore species representing eight carnivore families. The sequence and organization of the repetitive motifs can differ extensively between arrays; however, all motifs appear to be derived from the core motif "ACGT." Sequence data and Southern blot analysis demonstrate extensive heteroplasmy. The general form of the array is similar between heteroplasmic variants within an individual and between individuals within a species (varying primarily in the length of the array, though two clones from the northern elephant seal are exceptional). Within certain families, notably ursids, the array structure is also similar between species. Similarity between species was not apparent in other carnivore families, such as the mustelids, suggesting rapid changes in the organization and sequence of some arrays. The pattern of change seen within and between species suggests that a dominant mechanism involved in the evolution of these arrays is DNA slippage. A comparative analysis shows that the motifs that are being reiterated or deleted vary within and between arrays, suggesting a varying rate of DNA turnover. We discuss the evolutionary implications of the observed patterns of variation and extreme levels of heteroplasmy.