Introduction: Dichlorodiphenyltrichloroethane (DDT), a persistent organochlorine pesticide, breaks down slowly in soil, has a long half-life, and bio-accumulates in plants and animals. Although DDT was banned in the U.S. in 1972 and in Mexico in 2000, it continues to be used in other malaria-endemic areas. There is substantial animal evidence that DDT exposure in utero and during early postnatal life impacts neurodevelopment. However, few studies have examined these associations in humans. A North Carolina study found levels of DDE, a metabolite of DDT, to be dose-related to neonatal hyporeflexia (Brazelton Neonatal Behavioural Assessment Scale), but this was not confirmed in a smaller study from New York. The latter study also did not find an association with 6- and 12-month olds' performance on the Fagan Test of Infant Intelligence. However, in a small Spanish study (n=92), cord serum DDE was negatively associated with 13-month olds' cognitive, psychomotor, and social development (Bayley Scales of Infant Development, Griffiths Scales of Infant Development). Aim: The present study aims to investigate the relationship of in utero DDT/DDE exposure and neurodevelopment of children from primarily Mexican farmworker families in the Salinas Valley, California. Methods: Participants were drawn from the CHAMACOS project, a longitudinal birth cohort study of pesticides and other environmental exposures and the health of pregnant women and their children. This analysis includes approximately 350 singletons with a maternal blood collected during pregnancy (approximately 26 weeks gestation) and measured for p,p- DDT, p,p- DDE, and o,p- DDT by gas chromatography high-resolution mass spectrometry. Results: p,p -DDT, p,p -DDE, and o,p -DDT were detected in almost all samples. Geometric mean levels and 95% confidence intervals were: p,p -DDT = 19.7 ng/g lipid (16.7, 23.2); p,p -DDE = 1361 ng/g lipid (1201, 1542); and o,p -DDT = 1.6 ng/g lipid (1.4, 1.8). Crude analyses indicate a relationship between serum levels and some measures of mental and psychomotor development at different ages: Among 6-month olds, p,p -DDT was negatively associated with Bayley Psychomotor Development Index (PDI) (ß= -1.6 points per log10 unit increase, p =0.08), but not the Bayley Mental Development Index (MDI). Among 12-month olds, o,p -DDT p,p -DDT, and p,p -DDE were all negatively associated with PDI (ß= -2.4, p =0.04; ß= -2.6, p =0.01; and ß= -2.7, p =0.04, respectively). o,p -DDT and p,p -DDT were also associated with MDI at 12 months (beta= -2.0, p =0.02; ß= -1.2, p =0.10, respectively). Among 24-month olds there were no associations with PDI, but o,p -DDT was associated with MDI (ß= -2.3, p =0.06). We will determine whether these findings remain after adjustment for potential confounders. Conclusion: In summary, we find preliminary evidence for an association between in utero exposure to DDT and DDE and neurodevelopment in children, but the associations may vary with age.