The allelic variability of four dinucleotide microsatellites located in the HLA region (MOGc, D6S265, MIB, and TNFa) was analyzed in 67 individuals representing three Amerindian populations of the Argentine Gran Chaco: Toba, Wichi and Chorote. Genomic DNA was prepared from peripheral blood and DNA was extracted using the standard phenol-chloroform procedure. Alleles were identified by PCR, using an end-labelled reverse oligonucleotide primer (fluorescent 6 - Fam labeling). Despite the low number of samples studied, a high level of gene diversity was observed in each population and for each locus. Moreover, the mean number of alleles was 7.7, 5.3, 10.0, and 7.0 at loci MOGc, D6S265, MIB and TNFa, respectively. Differentiation tests between pairs of populations showed a clear differentiation between the Wichi and the other two groups. However, the proportion of the total genetic variability that is due to differences among populations, estimated by the Gst' index, was relatively low (6%). Almost all the genetic variation occurred at the intra-population level (96%). The high intra-populational genetic variation suggests the existence of an intensive gene flow among the Gran Chaco tribes. Historical information seems to confirm this result.