College biology courses commonly use diagrams to convey information. These visual representations are embedded in course materials with the expectation that students can comprehend and learn from them. Educational research, however, suggests that many students have difficulty understanding diagrams and the conventions (e.g., labels, arrows) they contain. The present study evaluates biology students' ability to comprehend scientific diagrams and the diagram characteristics that affect this comprehension. Participants were students in a physiology course who completed a multiple-choice test of diagram comprehension ability (DCA) (Cromley JG, Perez TC, Fitzhugh SL, Newcombe NS, Wills TW, Tanaka JC. J Exp Educ 81: 511-537, 2013). We coded the conventions used in each test diagram and used these codes to capture the diagram characteristics of conventions and complexity. Descriptive analyses examine students' ability to understand scientific diagrams and which diagram characteristics cause the most difficulty. We also compared groups with low and high DCA scores to evaluate how students at different levels of comprehension ability are affected by diagram characteristics. Results show relatively poor DCA; the average total test score was only 69.5%. The conventions used in a diagram also affected diagram comprehension, and results show students had the most difficulty comprehending diagrams using a letter or numbering system, where arbitrary letters/numbers were used to signify objects and diagrams using cut-outs that showed cross sections and magnified interior views. Additionally, students' comprehension was higher on diagrams with higher complexity (i.e., more types of conventions used), potentially indicating students are able to take advantage of the supports that different conventions provide. Implications for instruction are identified.