Background: Manganese is an essential trace element and common component of water, soil, and air. Prenatal manganese exposure may afect fetal and infantile neurodevelopment, but reports on in utero manganese exposure and infant neurodevelopment are rare. oBjective: Tis study was conducted to investigate a relationship between maternal blood manganese level and neurodevelopment of infants at 6 months of age. Methods: Data were obtained from the Mothers and Children’s Environmental Health (MOCEH) birth cohort study. The study population included 232 pairs of pregnant women and their infants at 6 months of age. Maternal blood manganese was measured at term, just before delivery. Mental and psychomotor development in infancy was assessed at 6 months of age using the Bayley Scales of Infant Development. The relationship between maternal blood manganese level and the mental and psychomotor development indexes (MDI and PDI) was estimated for manganese modeled as a linear and as a categorical variable and using penalized splines for nonlinear modeling. results: Mean ± SD maternal blood manganese concentration was 22.5 ± 6.5 μg/L. After adjustment for potential confounders, blood manganese was used as a continuous variable in a linear and nonlinear model. Associations between maternal blood manganese and MDI and PDI scores followed an inverted U-shape dose–response curve after adjustment for potential confounders, with lower scores associated with both low and high blood concentrations [MDI: likelihood-ratio test (LRT) p = 0.075, PDI: LRT p = 0.038]. Associations of both outcomes with increasing blood manganese shifted from positive to negative at concentrations of 24–28 μg/L in this cohort of term, normal birth weight children. conclusion: Although no cut-of point has been established to define manganese toxicity, both high and low blood manganese levels may be associated with neurobehavioral function in infants.