46 Mbya-Guaraní individuals, from various settlements in the province of Missions, were characterised in the polymorphisms that define the top 4 Amerindian maternal haplogrupos. The classification was carried out in accordance with the following steps: (a) DNA extraction from oral spinning, (b) PCR amplification of 4 mytochondrial DNA polymorphic regions, (c) digestion with restriction enzymes, and (d) electrophoresis in agarose gels. The results showed that almost half of the individuals studied (47.8 %) belong to A, with low incidence in the vast majority of South American Amerindian populations. The boundary B is present at a frequency of 32.6 %, while the lines C and D are equally represented at a frequency of around 9 %. Only one of the individuals studied could not be assigned to any of the 4 mitochondrial haplogrupos, it being estimated that this is a non-amerindian lyage, given the virtual absence of X lyage in the South American populations studied. It is concluded that the observed distribution, which is unusual for a South American Amerindian population, could be due to the action of random evolutionary forces (gene drift, founder effect) rather than to directional processes (selection, migration), given the low frequency of haplogroup A among neighbouring aboriginal populations, or still among the groups in the Amazon region from where Mbya-Guaraní seems to come from.