The authors observed a group of 4 eighth-grade students who conducted a series of inquiry-based science investigations over 6 weeks. The authors examined evidence of students' motivational beliefs, thoughtfulness, nature of their conceptual understanding, and changes in that understanding. The authors also used pre- and posttests of conceptual understanding. Marked variability in terms of the types of students' motivations was found. The variations were associated with differences in the quality of their thoughtfulness and in the development of their conceptual understanding. Evidence of student mastery-goal orientation appeared most strongly related to increased understanding.Key words: eighth-grade students; motivational beliefs, thoughtfulness, conceptual understanding; science investigation