Researchers are aware that grouping students by prior knowledge may result in moderate gains in intermediate grade students' mathematics achievement. Despite this research, many teachers continue to teach the way they were taught: one curriculum for all students regardless of students' readiness. Additionally, researchers have raised concerns about the effects of flexible grouping on students' self-esteem. Little research examined the effects of curricular enhancement and whole group instruction on student achievement. Further, less research linked ability grouping to the specific enhancement and differentiation of curriculum based on student prior knowledge. Therefore, the purpose of this study was to investigate the combined effects of grouping with appropriate curricular practices on intermediate students' mathematics achievement. A further purpose was to compare classrooms that featured whole class instruction but were distinguished by the type of curriculum implemented: regular textbook versus a modified or remodeled curriculum unit. A pretest-posttest, comparison group-experimental group design using a purposive sample of 31 teachers and their students (N = 645) from four diverse school districts was used in this study. Teachers implemented three different types of grouping practices (whole class, Joplin Plan, and Flexible Small Groups [FSG]) and two types of curricular practices (modified and differentiated). Repeated Measures Analysis of Variance was employed to investigate the effects of different grouping arrangements and appropriate curricular design on the treatment and comparison groups. Results indicated significant differences, F(5, 253)=40.988, p < .001 (ES = .42), between treatment groups exposed to an enhanced unit and the comparison groups after adjusting for grade level (4 or 5). Further, results indicated significant differences, F(11, 645)=55.816, p < .001 (ES = .52 for FSG, ES = .28 for Joplin), among curricular (modified or differentiated) and grouping (whole, between, and within-class) treatment groups after adjusting for grade level (4 or 5). Qualitative procedures were used to analyze data from self-report instruments, observers' reports, interviews, and focus groups with teachers and students. Results indicated that teachers and their students preferred the between and within-class grouping arrangements to their typical whole class grouping plan. Additionally, teachers and students enjoyed and were motivated by the enhanced or differentiated curriculum. (Contains 6 tables.)