Prior studies have shown that students have difficulty understanding the role of mutation in evolution and genetics. However, little is known about unifying themes underlying students' difficulty with mutation. In this study, we examined students' written explanations about mutation from a cognitive science perspective. According to one cognitive perspective, scientific phenomena can be perceived as entities or processes, and the miscategorization of processes as entities can lead to noncanonical ideas about scientific phenomena that are difficult to change. Students' incorrect categorization of processes as entities is well documented in physics but has not been studied in biology. Unlike other scientific phenomena that have been studied, the word "mutation" refers to both the process causing a change in the DNA and the entity, the altered DNA, making mutation a relevant concept for exploration and extension of this theory. In this study, we show that, even after instruction on mutation, the majority of students provided entity-focused descriptions of mutation in response to a question that prompted for a process-focused description in a lizard or a bacterial population. Students' noncanonical ideas about mutation occurred in both entity- and process-focused descriptions. Implications for conceptual understanding and instruction are discussed.