This study examines the effects of employing student group leaders on the motivation of group members during co-operative learning activities in a secondary school classroom in Turkey. The study was carried out in a period of eight weeks in biology classes during which "living things" and "ecology" topics were taught to a class of 45 students (Year 9, 14 to 15 years old) by using Jigsaw and STAD (Student Teams Achievement Divisions). Students were divided into groups of four and a student in each group was assigned as the group leader. Data were collected through interviews with group leaders and group members and through video recordings of one group continuously for eight weeks. The study revealed that student group leaders' influenced the motivation of the group members in different ways. These were called reward, relationship, role-model, emotion and learning- oriented motivational strategies. (Contains 1 figure and 2 tables.)