This paper is a report of a study that examines the relationship between teacher participation in a multi-year, K-6 professional development effort and the "high stakes" science test scores of different student groups in 33 rural mid-west school districts in the USA. The professional development program involved 1,269 elementary school teachers and utilized regional summer workshops and distance delivery technologies to help the teachers learn science concepts, inquiry teaching strategies, and how to "adapt" science inquiry lessons to teach and reinforce skills in the language arts. Regression analyses revealed a significant positive relationship between the professional development hours experienced by teachers and student gains on high stakes test scores. But the analyses also suggest that primary grade teachers need less professional development than upper grade teachers on instructional strategies where the focus is on the integration of language arts and science inquiry. The implications for funding and implementing professional development projects are discussed.