Introduction: Despite animal evidence suggesting neurotoxic effects of organophosphate (OP) pesticides, few published studies investigate the neurodevelopmental toxicity of OP pesticides in humans. This study investigates exposure to OP pesticides and neurodevelopment of infants from low-income Mexican farmworker families in California. Methods: Participants from this birth cohort study include 445 singletons with psychomotor (PDI) and mental (MDI) development assessed using the Bayley Scales of Infant Development at 6 (n = 405), 12 (n = 405), and/or 24 (n = 379) months of age. Six nonspecific dialkylphosphate (DAP) metabolites of OP pesticides were measured in urine collected from mothers twice during pregnancy (mean = 13 and 26 weeks gestation) and from children at the time of the neurodevelopmental assessments. The 6 DAPs were summed to obtain total DAPs at each time point, representing exposure to approximately 80% of OPs used in the Salinas Valley (California Environmental Agency, 2002 no. 27). Total DAPs were transformed to the log10 scale. Pregnancy DAPs were averaged before log10 transformation. Results: Median total DAPs levels were 103.2 and 111.8 nmol/g in pregnant mothers and 37.0, 54.4, and 75.6 nmol/g in children at ages 6, 12, and 24 months, respectively. Bayley MDI scores (mean ± standard deviation) were 95.6 ± 7.0, 100.8 ± 8.9, and 85.8 ± 11.7 at 6, 12, and 24 months, respectively; corresponding PDI scores were 96.4 ± 10.6, 106.0 ± 12.7, and 97.6 ± 10.6, respectively. We found a 3.51-point decrease in 24-month MDI (P = 0.02) with each 10-fold increase in average pregnancy DAPs, but no association at 6 and 12 months. We found a 1.49-point increase (P = 0.04) in 12-month MDI and a 1.97-point increase (P = 0.04) in 24-month MDI associated with each 10-fold increase in children's DAPs concurrent to the assessments. We observed no interaction between prenatal and concurrent levels, and no association between DAPs metabolites and PDI. Analyses using diethyl and dimethyl phosphate metabolites, which sum to make total DAPs, will also be presented. Analyses of DAPs and maternal reports of behavior from the Child Behavior Checklist results are ongoing and will also be discussed. Discussion and Conclusions: These results suggest that prenatal exposure, but not postnatal exposure, to OPs may be negatively associated with mental neurodevelopment during early childhood. We have no ready explanation for the positive associations we observe between children's DAPs metabolites and MDI, although we note that the outcomes may precede these exposures.