In this article, we examine classroom observations from a 3-year large-scale randomized trial in the Los Angeles Unified School District (LAUSD) to investigate the extent to which a professional development initiative in inquiry science influenced teaching practices in in 4th and 5th grade classrooms in 73 schools. During the course of the study, LAUSD introduced an additional districtwide scientific inquiry professional development initiative, which complicates the experimental analysis but allows us to conduct a quasiexperimental analysis of the second Multilevel models predicting the presence of science inquiry in observed classroom lessons show that both interventions increased the incidence of inquiry-based science teaching, but the impact was limited to selected features of the inquiry process. We also found that the experimental impacts on teaching practice correspond with the features of scientific inquiry to which the teachers were most frequently exposed during the professional development. (Contains 8 notes and 6 tables.)