Three hundred and forty-nine first to sixth graders (ages 6 years and 6 months to 12 years and 11 months) from two Mexican schools (one private and one public) were administered the Comalli-Kaplan version of the Stroop test in Spanish, as part of a larger protocol. Findings are discussed in the context of current interpretations of the Stroop test, considered a measure of inhibition. No gender differences were observed. Parental education was a statistically significant but unimportant predictor of performance. Developmental norms are provided, together with analyses of the amount of interference encountered (in time and errors) for each age.