The purpose of this research is to examine the impact of a Standards-based elementary mathematics curriculum on third grade students' mathematics performance. A total of 707 students participated in this study. Of this total, 368 students were from eight schools located within the same school district in a racially and ethnically diverse large city in the Midwest. Another 339 of the students were from four schools located in two different school districts in a middle- to high-SES, largely white, suburban area in the Northeast. Third grade students using "Investigations" curriculum outperformed in most cases and performed the same in the other cases matched comparison groups who were using a range of conventional curricula on the mathematics assessment. The Investigations groups did the same or better on the decontextualized, contextualized, and algebraic-reasoning constellations compared with their counterparts. The revised Investigations curriculum was not as effective with the low SES African-American students as it was for middle to high SES, white students. The curriculum fidelity measures did not differentiate achievement differences.