Long tracts (megatracts) of (CAG)n, (TAG)n, and (GAA)n microsatellite sequences capable of forming composite DNA segments were found in the maize (Zea mays L.) genome. Some of the (CAG)n and (TAG)n megatracts were organized in clusters of up to 1 Mb on several chromosomes, as detected by fluorescence in situ hybridization (FISH), as well as on extended DNA fibers. Extensive polymorphism was found among different maize inbred lines with respect to the number and size of microsatellite megatract clusters on the A chromosomes. Polymorphism was also common among B chromosomes of different nuclei in the inbred line Zapalote Chico. Different retrotransposable elements were often inserted into the microsatellite tracts. Size variation in some (TAG)n and (GAA)n megatracts was observed in consecutive generations among siblings of the inbred lines, indicating that these loci are highly unstable and predisposed to dynamic mutations similar to those described in mammalian systems.