Students' ability to participate in scientific discourse and to appropriately use empirical evidence to support beliefs or conclusions has been consistently stated as a core goal of science education. In the present study, we analyzed the quality of scientific reasoning in elementary school science classrooms, using the Evidence-Based Reasoning (EBR) Video Framework (see Furtak, Hardy, Beinbrech, Shavelson, & Shemwell, this issue). Two data sets from six 3rd-grade and 4th-grade science classrooms drawn from class discussions on floating and sinking were analyzed for the quality of EBR, the kinds of teacher prompts, and the level of conceptual understanding in classroom discourse. We found that the majority of discourse involved unsupported claims about the scientific phenomena. Although there was a clear progression in conceptual understanding over the course of the curriculum, no consistent effects were observed in the level of EBR. There was some evidence for effects of a preceding curriculum that had focused on nature-of-science constructs on the quality of students' EBR. Moderate correlations were obtained between students' conceptual understanding and reasoning level within reasoning units. Teacher prompts for providing support for conclusions and inferences were associated with higher reasoning levels, emphasizing the role of teachers in promoting a culture of productive use of evidence in classroom discourse. The quality of the EBR Video Framework as an assessment tool is discussed. (Contains 3 tables and 6 figures.)