While learning progressions (LPs) hold promise as instructional tools, researchers are still in the early stages of understanding how teachers use LPs in formative assessment practices. We report on a study that assessed teachers’ proficiency in using a LP for student ideas about hydrologic systems. Research questions were: (a) what were teachers’ levels of proficiency for using LP-based formative assessment practices and (b) how were teachers’ proficiencies evident in the use of these practices in their classrooms? Drawing on written assessments, item response theory was employed to assess proficiency in the practices of setting learning goals, interpreting student ideas, and responding to ideas with instruction. In addition, two case studies provided examples of how teachers used the practices. Most teachers adopted a didactic approach, emphasizing interpreting ideas as right or wrong and identifying goals and responses focused on transmission of factual science ideas. This study provides insights concerning how science teachers adopt LP-based formative assessment practices and can inform the design of professional development and curriculum materials to support teachers in enacting LP-based instruction.