This study aimed at comparing the effectiveness of problem-based learning (PBL) 1 and traditional lecture-based instruction on elementary school students' academic achievement and performance skills in a science unit on genetics while controlling for their reasoning ability. For the specified purpose, two instructional methods were randomly assigned to intact classes of two different teachers. Each teacher had both PBL classes and traditional classes. Although students in PBL classes (n = 126) worked on ill-structured problems cooperatively with the guidance of the teacher, students in traditional classes (n = 91) received instruction based on teacher's explanations, discussions, and textbooks. Genetics Achievement Test was developed by researchers to measure the academic achievement and performance skills. Multivariate Analysis of Covariance results showed that the PBL students had higher academic achievement and performance skills scores (M = 11.44 and M = 2.67, respectively) when compared with those in traditional classes (M = 10.91 and M = 2.20, respectively). This indicated that the PBL students tend to better acquire scientific conceptions related to genetics and integrate and organize the knowledge. Moreover, it was found that the reasoning ability explained a significant portion of variance in the scores of academic achievement and performance skills.Keywords: Problem-based learning, elementary school, genetics, academic, performance.Problem-based learning (PBL) is an instructional approach that allows students to interact with their environment while dealing with ill-structured problems. In PBL classrooms, knowledge evolves through social negotiation. Furthermore, problems with no clear-cut solution encourage students to take alternative point of views and strategies into consideration and apply their newly constructed knowledge into new areas [1][2][3]. In fact, through addressing problems with no single, correct solution, students learn how to assess what they know, identify what they need to know, gather information, and collaborate on the evaluation of hypotheses and ideas based on the data they collected. As students attempt to deal with the problem, the nature and definition of the problem may change. Teachers serve as facilitators, and students are only given guidelines for approaching the problems. Students take responsibility for what is learned and how [4].PBL was first introduced in the medical schools and has had a major impact on thinking and practice in medical education for the past 30-40 years. This approach has been based on active learning in small groups, with clinical problems used as the stimulus for learning, and it has been evaluated in medical literature for its ability to incorporate realistic experiences in the classroom [5][6][7][8][9][10][11][12]. Apart from the studies conducted in the field of medical education, there are studies in the literature that aimed at adapting PBL for use in elementary and high school settings [13][14][15][16][17][18][19]. Results, in general, rev...

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