The genetic variability of butyrylcholinesterase, determined by the BCHE and CHE2 loci, was examined in nine Brazilian Indian groups. In addition, a search for the presence of the BCHE*F allele was also performed in eight other Brazilian Indian samples and in five admixed (black-Indian-white) rural Amazonian communities previously studied for the CHE2 locus and the BCHE*A allele. In the Indian populations the frequency of the BCHE*F allele varied from 0 to 7.1% +/- 3.4 and the frequency of the CHE2 C5+ phenotype ranged from 1.4% +/- 1.4 to 45.9% +/- 3.8. This study seems to be the first to report the presence of the BCHE*F allele in native Americans. The BCHE*A allele appeared in one Indian group (1.4% +/- 1.0), and we suggest that its existence in this tribe and in other native Americans can be explained by gene flow from white populations. Gene flow may also be the reason for the occurrence of the BCHE*F allele in Brazilian Indians, whereas the CHE2*C5+ allele may have been present in the paleo-Indians. The distributions of both the BCHE*F allele and the CHE2 C5+ phenotype in Brazilian Indians seem to be the result of the action of random genetic drift.