In this study, the effect of problem-based learning on students' academic achievement and performance skills in a unit on the human excretory system was investigated. Sixty-one 10th grade students, from two full classes instructed by the same biology teacher, were involved in the study. Classes were randomly assigned as either the experimental or the control group and were pre- and post-tested to determine their academic achievement and performance skills before and after the treatment. The experimental group was taught with problem-based learning while the control group received traditionally-designed biology instruction. Results showed that although there was no pre-existing difference between two groups, students instructed with problem-based learning earned significantly higher scores than those instructed with traditionally-designed biology instruction--in terms of academic achievement and performance skills. Students in the experimental group appeared to be more proficient in the use and organisation of relevant information, in constructing knowledge and moving toward better conclusions. (Contains 3 tables.)