This study explores teachers' informal formative assessment practices in three middle school science classrooms. We present a model for examining these practices based on three components of formative assessment (eliciting, recognizing, and using information) and the three domains linked to scientific inquiry (epistemic frameworks, conceptual structures, and social processes). We describe the informal assessment practices as ESRU cycles--the teacher Elicits a question; the Student responds; the teacher Recognizes the student's response; and then Uses the information collected to support student learning. By tracking the strategies teachers used in terms of ESRU cycles, we were able to capture differences in assessment practices across the three teachers during the implementation of four investigations of a physical science unit on buoyancy. Furthermore, based on information collected in a three-question embedded assessment administered to assess students' learning, we linked students' level of performance to the teachers' informal assessment practices. We found that the teacher who more frequently used complete ESRU cycles had students with higher performance on the embedded assessment as compared with the other two teachers. We conclude that the ESRU model is a useful way of capturing differences in teachers' informal assessment practices. Furthermore, the study suggests that effective informal formative assessment practices may be associated with student learning in scientific inquiry classrooms.