The purpose of the present study was to examine gender differences in mathematics interest. In a sample of middle school students (N = 366), we examined mathematics interest as a trait (i.e., individual interest) and as a current state (i.e., situational interest) during a learning activity in mathematics. Assessment of perceived competence, performance, and effort during learning was also included. Results revealed a significant difference between boys and girls in their ratings of individual interest, but not in the current experience of situational interest. Furthermore, boys and girls did not differ in performance during learning, but girls invested significantly more effort than boys. Results and their implications are discussed in relation to interest development, gender differences, and subjective beliefs.