This article reports an investigation of a professional development program to enhance elementary teachers' ability to engage their students in argument from evidence in science. Using a quasi-experimental approach, three versions were compared: Version A--a 1-week summer institute with a 2-week summer practicum experience and 8 follow-up days (four per year), Version B without the practicum experience, and Version C--a revision of Version A in Year 3. All teachers were videoed twice each year, and the videos were rated using an instrument to measure the quality of discourse. All versions led to a significant improvement in teachers' facilitation of classroom discourse. Neither the practicum nor the revised program had an additional effect. Implications for the field are discussed.