Ainsworth's (2006) DeFT framework posits that different representations may lead learners to use different strategies. We wanted to investigate whether students use different strategies, and more broadly, different cognitive activities in diagrams vs. in running text. In order to do so, we collected think-aloud protocol and other measures from 91 beginning biology majors reading an 8-page passage from their own textbook which included seven complex diagrams. We coded the protocols for a wide range of cognitive activities, including strategy use, inference, background knowledge, vocabulary, and word reading. Comparisons of verbalizations while reading running text vs. reading diagrams showed that high-level cognitive activities--inferences and high-level strategy use--were used a higher proportion of the time when comprehending diagrams compared to when reading text. However, in running text vs. diagrams participants used a wider range of different individual cognitive activities (e.g., more different types of inferences). Our results suggest that instructors might consider teaching students how to draw inferences in both text and diagrams. They also show an interesting paradox that warrants further research--students often skipped over or superficially skimmed diagrams, but when they did read the diagrams they engaged in more high-level cognitive activity. (Contains 2 figures and 4 tables.)