Seeking improved student performance in elementary schools has led educators to advocate inquiry-based teaching approaches, including problem-based learning (PBL). In PBL, students simultaneously develop problem-solving strategies, disciplinary knowledge bases, collaborative skills, and dispositions. Research into the efficacy of PBL in elementary school settings is in the seminal stage and reveals mixed results. In this pilot study, 4th graders receiving PBL in science were compared with a corresponding group receiving the same instruction in thematic format. Using a quasi-experimental design, the researchers investigated students' knowledge of content, stereotypical images of scientists, time-on-task, and transfer of problem-solving skills. (Contains 7 tables.)