Background: Mercury release from gold mining activities and contamination of riverine populations is a matter of growing concern in the Beni basin (Lowland Bolivia). Objectives: To document mercury contamination of Amerindian populations living along the Beni River (Bolivia) and to examine risk factors related to their lifestyle. Design: A cross-sectional survey performed among 15 communities on the flood plains of the Beni River at the foot of the Andes. A total of 631 subjects were examined (173 mothers, 393 children 0-10 years old and 104 adolescents). Methods: Hair mercury content (H-Hg) served as a bio-indicator of mercury contamination. After digestion with EDTA, mercury content was measured using atomic absorption spectrometry with cold vapor generation. Four indicators of lifestyle were analyzed: community accessibility (road or canoe); subsistence activity (including or not fishing); fish consumption (number of servings and fish species); and ethnicity (member of the Tacana or EseEjjas ethnic group). Results: The median of H-Hg was equal to 4.0 μg/g (IC95%: 3.6 ~ 4.4). About 86% of the subjects had H-Hg values lower than 10 μg/g. There were no significant differences in H-Hg between adult women and children, nor according to age group. However, there was a decrease in H-Hg from birth to 18 months and an increase thereafter. Subjects belonging to the Esse-Ejja ethnic group had higher H-Hg than the Tacanas. Communities accessible only by canoe were more frequently contaminated than those accessible by road. Subjects who ate at least one serving of fish per day had higher H-Hg, and families who maintained substantial fishing activity were more strongly contaminated. Logistic regression analysis showed an interaction between fish consumption and village accessibility or fishing activity, explaining the differences between the 2 ethnic groups. Conclusions: Contamination levels were found to be low compared with other Amazonian studies. However, the most strongly affected groups were those which preserved a traditional way of life and were the most economically and socially disadvantaged.