The relative development of Piagetian concrete logical operational abilities was investigated in Mexican American and Anglo American children. Performance was assessed on eight classification tasks and three forms of transitivity, measuring seriation skills. One phase of the study compared the performances of 30 Anglo and 30 Mexican American students from a lower-middle class school, grades K-3. Ethnic background, age, grade, and sex were not found to be effective predictors of the measured abilities. Verbal ability was the best single predictor of classification task performance. Another sample of younger students from a middle class school scored at a higher level than the total group, indicating socioeconomic status to be an important factor. (Author/MV)