This study examined the impact of the 3-year implementation of a professional development intervention on science achievement of culturally and linguistically diverse elementary students. Teachers were provided with instructional units and workshops that were designed to improve teaching practices and foster positive beliefs about science and literacy with diverse student groups. The study involved third, fourth, and fifth grade students at six elementary schools in a large urban school district during the 2001 through 2004 school years. Significance tests of mean scores between pre- and posttests indicated statistically significant increases each year on all measures of science at all three grade levels. Achievement gaps among demographic subgroups sometimes narrowed among fourth grade students and remained consistent among third and fifth grade students. Item-by-item comparisons with NAEP and TIMSS samples indicated overall positive performance by students at the end of each school year. The consistent patterns of positive outcomes indicate the effectiveness of our intervention in producing achievement gains at all three grade levels while also reducing achievement gaps among demographic subgroups at the fourth grade. (Contains 10 tables.)