This study aims to determine the effects of cooperative learning (using the jigsaw method) on students' achievement in a general chemistry course. The Chemistry Achievement Test (CAT) and Phase Achievement Test (PAT) were used. The questions on the CAT relate to solids, liquids, gases, bonding, matter, and matter states. This test was given to students who were not participating in the study although they had previously taken the course, and had studied the topics listed above. The reliability coefficient for this test was 0.79. The PAT, developed by the author and three chemistry teachers, has four modules; each module consists of four multiple-choice questions. This study included a total of 108 chemistry students in two different classes during the 2004-2005 academic year. One of these classes served as the experimental group (n = 52), which was taught using cooperative learning (jigsaw) methods, while the other class served as the control group (n = 56), which was taught using traditional learning methods. The results indicate that the instruction based on cooperative learning yielded significantly better achievement in terms of the CAT and PAT scores compared to the test scores of the control group, which was taught with traditionally designed chemistry instruction. (Contains 3 tables and 1 figure.)