This article reports on the results of a planning grant studying the effects of a highly rated curriculum unit on a diverse student population. The treatment was introduced to 1500 eighth grade students in five middle schools selected for their ethnic, linguistic, and socioeconomic diversity. Students were given pre-, post-, and delayed posttests on a Conservation of Matter Assessment and measures of motivation and engagement. This quasi-experiment found statistically significant posttest results for achievement, basic learning engagement, and goal orientation. Analyses of disaggregated data showed that subgroups of students in the treatment condition outscored their comparison group peers (n = 1500) in achievement in all cases, except for students currently enrolled in ESOL. Analysis of video data of a diverse group of four students as the unit was enacted suggests that students entered a learning environment that permitted them to function in different, but consistent ways over time; that is, the frequency of students' manipulation of objects showed a different pattern of engagement for each of the four students compared with patterns of verbal responses such as the use of scientific terms. The results of this planning grant paved the way for a large study of the scale-up of highly rated curriculum units. © 2005 Wiley Periodicals, Inc. J Res Sci Teach 42: 912–946, 2005