Student evaluation of teaching (SET) is employed as an aid in improving instruction and determining faculty teaching effectiveness. However, economic theory indicates the existence of externalities in SET scores that directly influence their interpretation. As a test of this existence, a multinomial-choice, ordered data estimation procedure is employed to identify course externalities influencing SET. These externalities include student class standing, required courses, class size, days a class meets, class meeting time, classroom location, and classroom design. Results indicate that externalities have a significant impact on teaching evaluations. Thus, failure to internalize these externalities will lead to biases in SET and questionable use of SET as an aid in instruction improvement and determining faculty effectiveness.