The achievement gap in mathematics between African American and Caucasian students has been observed for several decades, though previous research has been unable to fully explain why the gap exists. Using a sample of 56 kindergarten and first grade students, this study examined the role that cognitive developmental level plays in the achievement gap. Results indicated that African American students scored significantly lower than the Caucasian students on a test of mathematical achievement. Furthermore, proportionally more African American students remained in the preoperational stage of development as compared to their same-age Caucasian peers. Finally, after controlling for cognitive developmental level, the achievement gap between the two groups became statistically indistinguishable, suggesting that cognitive developmental level mediates the relationship between ethnicity and mathematic achievement.