The aim of the present study was to investigate how situational interest develops over time and how it is related to academic achievement in an active-learning classroom. Five measures of situational interest were administered at critical points in time to 69 polytechnic students during a one-day, problem-based learning session. Results revealed that situational interest significantly increased after the problem stimulus was presented. Subsequently, situational interest gradually decreased but at the end of the day increased again. Testing a path model relating the situational interest measures showed strong (directional) interrelations. Moreover, situational interest was highly predictive for observed achievement-related classroom behaviors. The latter, in turn, proved to be a significant predictor of academic achievement. Aggregating situational interest over the day led to less accurate predictions of achievement-related classroom behaviors and academic achievement. Implications of these findings for situational interest research are discussed. (Contains 1 table and 2 figures.)