Students' interest in science declines substantially in the transition from elementary to secondary education. Using students' ratings of their instruction on the topic of evaporation and condensation, we examined if changes in instructional practices accounted for differences in situational interest in science instruction and enduring individual interest in science between elementary and secondary school classrooms. Multilevel regression analyses were conducted for a sample of 60 fourth- and 54 sixth-grade classrooms. The use of student experiments, the elicitation of student explanations, and lack of clarity accounted to varying degrees for disparities in science interest between grade levels. The impact of instructional practices on individual interest was mediated by situational interest. This corroborated predictions of the person-object theory of interest.