Intelligence is a complex behavioral phenomenon, the biological substrates for which require further investigation. The present study investigated differences in electroencephalographic patterns of P300 waves between average- and high-intelligence Brazilian children using a cognitive working memory task. Brain activity was recorded in 8 children (Mage = 11.2 years, SD = .40 years) while they performed a working memory task. Four children had an IQ > 120 (high-IQ group), and 4 had an IQ between 90 and 110 (average-IQ group). Different event-related potential (ERP) components were analyzed. The latency and amplitude of these wave peaks were analyzed by repeated-measures analysis of variance. The groups differed in reaction time but not in the level of accuracy of their responses. With regard to ERPs, we observed an effect of intelligence on P3 latency, the difference for which reached the greatest magnitude, but this effect was only marginally significant (p = .098). Differences in the amplitudes of P2 and P3 were not significant between groups. Likewise, the more intelligent children were more likely to respond to the behavioral task more quickly. They also had a lower latency of the later components of ERPs. Although the results were only marginally significant, they are consistent with previous studies in this field.