Mercury is an important source of environmental contamination affecting human beings throughout the world and especially in the Amazon. Riverside populations have been chronically exposed to relatively high levels of methylmercury for many years. Long-term effects of mercury exposure are not well known, but human genotoxicity was already showed in both in vitro and epidemiological studies. However, to date, only two studies were carried out in Amazonian populations with conflicting results and without comparing to a non-exposed population. Aiming to highlight this question and avoid interference factors, this work analyzed in vitro genotoxicity of mercury in blood lymphocytes of Amazonian individuals by two methods (micronucleus and chromosomal aberrations). Deleterious effects of low levels (1-500 μg/l or 0,004-2 μM) of methylmercury were only detected with the method to detect chromosomal aberrations. Mitotic index (proportion of cells in metaphase) was the parameter most sensible. Thus, this technique was applied for the analysis of an Amazonian non-exposed population (Panacauera) with similar social-economical characteristics of the exposed populations studied elsewhere. The mean of the mitotic index for Panacauera population was 0.0814 ± 0.0097. Inter-individual variability of this index had no relation with sex or age. This value was above those registered for some groups of exposed populations. This fact points to mercury as the main responsible for inhibiting the cell cycle and/or the loss of proliferative capacity of the cells. These results already support mitotic index as an essential parameter for the early diagnose of mercury genotoxicity in humans, and especially in Amazonian populations.