Graduate teaching assistants (GTAs) constitute a valuable and economical teaching force in many higher education undergraduate programmes. However, student satisfaction with their teaching has attracted little attention in the research literature. This study aimed at examining students' evaluation of teaching of GTAs in discussion groups, as well as exploring the effects of group and GTA variables on these ratings. Data were collected using a questionnaire administered online and completed by 7078 undergraduate students. Participants were enrolled in classes taught by 278 GTAs from four faculties in a major Israeli university. Results indicated that ratings assigned to clarity of instruction were the most salient predictor of students' overall evaluation. Generally, findings were consistent with those reported in the literature for other categories of instructors. Groups taught by GTAs in exact sciences and engineering were rated higher than those in social sciences and business management. Group size and the percentage of men students were inversely correlated with student ratings, while student attendance rate was positively correlated. Women GTAs and GTAs who taught more than one group tended to receive higher ratings. Overall student attendance rate was the most prominent predictor of student ratings. The implications of the findings are discussed.