This study investigated how the online problem based learning (PBL) approach employed in an online learning environment influenced undergraduate students' critical thinking skills (CTS) and content knowledge acquisition. The pretest-posttest control group design was used in the study. The subjects included the students who were enrolled at the Department of Primary School Mathematics Teaching in Anadolu University Education Faculty. Subjects attended to Computer If course in 2008 spring. Experiment group attended the online PBL course whereas the control group attended the online instructor-led course. Each group consisted of 20 students. Data collection tools consisted of a multiple choice content knowledge acquisition scale and the Watson-Glaser critical thinking skills test. The results of two-way mixed design ANOVA indicated that learning in the online PBL group did not have a significant effect on the content knowledge acquisition scores. It was also revealed that learning in the online PBL group had a significant effect on increasing the critical thinking skills. (C) 2009 Elsevier Ltd. All rights reserved.