Context: Organophosphate (OP) pesticides are neurotoxic at high doses. Few studies have examined whether chronic exposure at lower levels could adversely impact children’s cognitive development. Objective: To examine associations between prenatal and postnatal exposure to OP pesticides and cognitive abilities in school-age children. Methods: We conducted a birth-cohort study (CHAMACOS) among predominantly Latino farmworker families from an agricultural community in California. We assessed exposure to OP pesticides by measuring dialkyl phosphate (DAP) metabolites in urine collected during pregnancy and from children at age 6 months and 1, 2, 3½ and 5 years. We administered the Wechsler Intelligence Scale for Children-IV to 329 seven-year old children. Analyses were adjusted for maternal education and intelligence, HOME score, and language of cognitive assessment. Results: Urinary DAP concentrations measured during the 1st and 2nd half of pregnancy had similar relations to cognitive scores, thus we used the average of concentrations measured during pregnancy in further analyses. Averaged maternal DAP concentrations were associated with poorer scores for Working Memory, Processing Speed, Verbal Comprehension, Perceptual Reasoning, and Full Scale IQ. Children in the highest quintile of maternal DAP concentrations had an average deficit of 7.0 IQ-points compared with those in the lowest quintile. However, children’s urinary DAP concentrations were not consistently associated with cognitive scores. Conclusions: Prenatal but not postnatal urinary DAP concentrations were associated with poorer intellectual development in 7-year-old children. Maternal urinary DAP concentrations in the present study were higher, but nonetheless within the range of levels measured in the general U.S. population.