Teachers' implementation of a problem-based learning (PBL) program was examined to determine both how they assessed student learning and their reasons for these assessment practices. Ten 6th grade science teachers used Alien Rescue, a computer-based PBL module, with their students for approximately three weeks. Interviews, observations, and teacher-developed artifacts were analyzed qualitatively. The results suggest that assessment-related issues impacted the way that teachers implemented this PBL program, with teachers using grades as extrinsic motivators, deemphasizing PBL' usual focus on student reflection on their process and solutions, and adding assignments to the PBL program in order to provide greater structure for students' process and help prepare them for standardized tests. Some of these adaptations conflict with practices typically advocated in the literature on PBL. The implications of these results for the design of PBL programs are discussed, with an emphasis on supporting teachers' (a) local adaptations of PBL materials and (b) professional development through educative components embedded in PBL programs.