The "Framework for K-12 Science Education" (National Research Council, 2012) outlines eight practices to represent the diverse ways scientists construct and evaluate knowledge. Engaging students in these practices is a key instructional target in the science classroom. This target, however, creates particular challenges for online instruction, which has predominantly focused on delivering content. This study shows the possibility of addressing disciplinary practices online, here in the context of a professional development course for in-service science teachers, which we designed based on previous work in "responsive teaching." We examine an episode of the participants' engagement in an online text-based message board and identify evidence of disciplinary practices in their work. We discuss design elements and instructional choices that supported disciplinary engagement: building instruction around learners' ideas, developing online spaces in interaction with learners, and privileging learners' engagement in disciplinary practices as an objective.