BACKGROUND: Methylmercury (MeHg) is a known developmental neurotoxicant present in fish, yet the impact of childhood exposure is uncertain, especially in children with poor nutrition. OBJECTIVES: To evaluate associations of MeHg exposure with cognitive abilities and behavior in children from a poor rural area in Bangladesh. METHODS: The MeHg exposure was assessed in 1434 ten-year-old children by measuring mercury concentrations in hair (hair-Hg), using inductively coupled plasma mass spectrometry. Cognitive abilities were assessed with the Wechsler Intelligence Scale for Children (4th edition), and behavior by the parent-report Strengths and Difficulties Questionnaire. RESULTS: Children’s median hair-Hg was 674mug/kg (range 70-17,561mug/kg). Concentrations did not differ by distance from scalp, indicating a similar exposure over time. No multivariable-adjusted associations were observed between the children’s hair-Hg and their cognitive abilities. However, children in the highest tertile of hair-Hg had a lower prevalence of hyperactivity (OR: 0.61, 95% CI: 0.45-0.83) and peer relationship problems (OR: 0.58, 95% CI: 0.44-0.76), compared to children in the lowest tertile. The corresponding ORs were slightly stronger in children from families with lower socioeconomic status (OR: 0.56, 95% CI: 0.34-0.90 and OR: 0.48, 95% CI: 0.30-0.74) than in children from families with higher socioeconomic status (OR: 0.73, 95% CI: 0.49-1.08 and OR: 0.72, 95% CI: 0.51-1.02). Similar stronger ORs were observed in underweight children. CONCLUSION: MeHg exposure was not associated with cognitive function, but inversely associated with certain adverse behavior. More studies are needed concerning the risk-benefit of different fish consumption in rural Bangladesh.