Argumentation in science is the process of coordinating theory and evidence to justify conclusions. This practice is at the heart of scientific journal writing and communication, but little is known regarding the argument quality of college science majors, the future scientists. Studies on written arguments at the college level have focused primarily on non-majors and upper-level students. To investigate and describe these skills throughout the biology curriculum, majors ("n" = 243) in four levels of undergraduate biology courses at a public university were assessed using a short, written argument instrument based on a hypothetical data set and scenario. Using Toulmin's argumentation pattern to assess the instruments for argument aspects and quality, very few differences were found in the scores across course levels. Students were able to generate simple arguments consisting of claim, evidence and reasoning. The ability to provide scientific principles as reasoning to connect evidence and claims was positively correlated with course level. However, advanced argumentation skills, such as creating alternative explanations and rebuttals, were lacking across all course levels. These findings imply the need for explicit attention to argument construction throughout the undergraduate biology curriculum.