As a model to examine the local distribution of sequence variation within large arrays of tandemly repeated DNA in complex genomes, the long-range organization of alpha-satellite DNA from human chromosome 17 was investigated. Three individual chromosomes, representing different alpha-satellite haplotypes, were segregated into mouse and human somatic cell hybrids and their arrays sized by pulse-field gel electrophoresis. An inventory of the higher-order repeat units found in multiple separate regions of these megabase arrays was obtained using cosmid mapping and two-dimensional gel electrophoresis, a technique that combines the large-scale resolution of pulsed-field gel electrophoresis with the small-scale resolution of conventional gel electrophoresis. These analyses show that alpha-satellite arrays are characterized by the presence of localized homogeneous domains containing only one distinct type of repeat unit. These domains, which consist of sequence variants and/or higher-order repeat length variants, can be up to at least several hundred thousands of bases in length. Both abundant and rare variant repeat units can be localized in these distinct domains, which may correspond to transition states in the evolution of tandem multicopy DNA families. This description of the organization of large arrays of tandem repeats provides insight into mechanisms involved in their homogenization.