What does informal formative assessment look like in the context of scientific inquiry teaching? Is it possible to identify different levels of informal assessment practices? Can different levels of informal assessment practices be related to levels of student learning? This study addresses these issues by exploring how 4 middle school science teachers used questions as a method of informal formative assessment, and compares those practices to measures of student learning. The approach to exploring each teacher's questioning practices is based on viewing whole-class discussions as assessment conversations in which the teacher has the opportunity to draw out and act on students' evolving understanding. Assessment conversations are described as consisting of four-step cycles, where the teacher elicits a question, the student responds, the teacher recognizes the student's response, and then uses the information collected to student learning. Our results indicate that the teachers whose enactment of informal formative assessment was more consistent with this model had students with higher performance on embedded assessments. This trend was also reflected in the posttest scores. In addition, we found that teachers focused more on epistemic, rather than conceptual, features of scientific inquiry in their discussions. The study underlines the importance of informal formative assessment during scientific inquiry discussions for teacher training and professional development as a way to increase student learning.