Given the widespread adoption of the Next Generation Science Standards in the United States, there is a need to study new models of professional development to prepare teachers to enact practice-based visions of science learning and to address problems of access to reach teachers nationwide. While studies of online professional development offer promise in addressing both of these challenges, there is still much to be learned about how to design and orchestrate these learning environments. In particular, researchers have highlighted instructor facilitation as critical for teacher learning and reflection, yet there is a need for greater specification of online facilitation and how it can achieve targeted professional development goals. In this paper we explore responsive teaching as an approach to facilitating upper elementary and middle school teachers' scientific engagement in asynchronous online learning environments. Using a case study approach, we examine instructors' facilitation and teachers' participation over four weeks in an online threaded discussion forum. This episode was situated within a 12-month hybrid-online professional development program, which had an explicit goal of supporting teachers to experience "doing science" themselves. Building on our prior work showing that the program was successful in fostering teachers' engagement in scientific practices online, we characterize the responsive nature of instructors' facilitation and how it supported teachers' individual and collective inquiry. Our analysis provides an existence proof of a responsive approach to facilitating scientific engagement online and offers insights into the design of online professional development that supports teachers in "doing science."