This study presents the findings of an analysis of evolutionary diagrams found in 31 biology textbooks for students ranging from middle school to the undergraduate level. Since the early 1990s, cladograms have found their way into high school biology textbooks, yet we know little about their effectiveness as interpretive and instructional tools in biology education. In this article we document the frequency and types of cladograms found in 31 textbooks, and classify and survey the other types of evolutionary diagrams used in the texts. Although cladograms comprised approximately 72 percent of the diagrams overall, we found virtually no attempt to explain their structure and theoretical underpinnings. Various other noncladogenic evolutionary diagrams, comprising 28 percent of the total, were distributed throughout all textbooks studied. On the basis of our analysis, we conclude that many of these evolutionary diagrams are confusing and may reinforce alternative conceptions of macroevolution. Biology educators should therefore recognize these problems and take measures to ameliorate their effects.