The authors used the Motivated Strategies for Learning Questionnaire to investigate the effectiveness of problem-based learning (PBL) and traditional instructional approaches on various facets of students' self-regulated learning, including motivation and learning strategies. Participants included 61 tenth-grade students from 2 intact classes instructed by the same biology teacher. The authors randomly assigned 1 class as the experimental group and the other class as the control group. Teachers instructed the control group with teacher-centered, textbook-oriented traditional instruction; they taught the experimental group with problem-based learning, in which students worked with ill-structured problems. Results revealed that PBL students had higher levels of intrinsic goal orientation, task value, use of elaboration learning strategies, critical thinking, metacognitive self-regulation, effort regulation, and peer learning compared with control-group students. (Contains 4 tables and 2 figures.)