Recent reforms in US science instruction have encouraged teachers to engage students in the process of inquiry, but have not given clear direction on the extent to which students should, or should not, be guided toward particular conclusions. This paper is a multiple-method exploration of the nature of guidance four middle school teachers provided during discussions during a physical science unit about density. Pre and posttests of students’ conceptual understanding identified teachers whose students had high and low learning gains through the course of the unit. Videotapes of each teacher’s lessons were then coded according to a framework of authoritative, dialogic, and blended guidance, as well as the conceptual levels addressed during discussions. Results indicate that the teachers whose students had higher learning gains at the end of the unit shifted more often between authoritative and dialogic communicative approaches, and led discussions which more closely mapped onto the expected conceptual progression underlying the unit. The study highlights the importance of actively shifting guidance to develop students’ conceptual understanding during inquiry-based teaching.