Overall interest in science has been argued to drive learner participation and engagement. However, there are other important aspects of interest such as breadth of interest within a science domain (e.g., biology, earth science). We demonstrate that intensity of science interest is separable from topic breadth using surveys from a sample of 600 middle school students. We also show that these two dimensions contribute differently to learning-relevant behavioral tendencies. Specially, regression analyses show: (1) that intensity of interest predicts both self-reported science classroom engagement and preferences to participate in optional science learning; and (2) that breadth of interest predicts science choice preference, but not science classroom engagement. These findings have implications for the conceptualization of interest, the measurement of interest, and practical applications for educators.