This study examined the impact of an intervention designed to promote inquiry-based instruction among early childhood/elementary preservice teachers in Earth science. Preservice teachers participated in training sessions and community-based internships to deepen Earth science content knowledge and develop inquiry-based practices. Analyses of Earth science content knowledge pre- and posttests reveal mixed results: four of the eight participants' posttest scores increased over the duration of the study, two participants' posttest scores remained the same, and two participants' posttest scores decreased. Results of the Science Teacher Inquiry Rubric (STIR) (Bodzin & Beerer, 2003) also show mixed results with four participants who exhibited student-centered practices and four participants who engaged in teacher-centered practices. Two preservice teachers with different profiles were selected for in-depth case studies. Findings reveal the importance of preservice teachers' conceptions of science inquiry and the school environment. We concluded that appropriate conceptions and supportive environments are prerequisite to sustaining prospective teachers' inquiry-based practices. (Contains 3 tables.)