In recent years, many animal species have been shown to exhibit a variable number of tandem repeats (VNTR) in the control region of their mitochondrial DNA. To characterize the nucleotide polymorphism of a VNTR system, this region was sequenced in individuals from two species of crickets (genus Gryllus) collected at a mainland locality and an island locality within each species. The data reveal a clear pattern of concerted evolution: homogeneity among repeats within individuals and populations, but heterogeneity among the tandem arrays from divergent populations, and between species. The patterns of nucleotide polymorphisms within the arrays show several instances of partial homogenization where derived nucleotides have not swept through all the repeats in the array. These serve as examples of concerted evolution "caught in the act". Additional repeat associated polymorphisms (RAPs) shed light on the molecular basis of the insertion and deletion of repeat units and, with the concerted evolution of VNTRs, provide phylogeographic resolution of island and mainland populations within and between species. The polymorphisms within VNTRs also provide a simple system in which to examine the balance of mutation and drift and offer a unique view of the histories of genomic and organismal phenomena in the patterns and processes of molecular evolution.