Three sets of genetic markers (blood group plus protein polymorphisms, mitochondrial DNA, and Y-chromosome) were compared in four French Guiana and one Brazilian Amerindian populations. Spearman's rank correlation coefficient between five gene diversity statistics and historical or present-day population sizes showed significant values, indicating loss of diversity due to population bottlenecks. The three sets of markers furnished distinct admixture estimates, and the blood group plus protein polymorphisms could have overestimated the European contribution to their gene pool. Correspondence analysis distinguished the coastal from the interior populations, possibly reflecting past migration events. Am. J. Hum. Biol., 2009. © 2008 Wiley-Liss, Inc.