Student evaluation of teaching (SET) is employed as an aid in improvinginstruction and determining faculty teaching effectiveness. However,economic theory indicates the existence of externalities in SET scores thatdirectly influence their interpretation. As a test of this existence, amultinomial-choice, ordered data estimation procedure is employed toidentify course externalities influencing SET. These externalities includestudent class standing, required courses, class size, days a class meets,class meeting time, classroom location, and classroom design. Resultsindicate that externalities have a significant impact on teachingevaluations. Thus, failure to internalize these externalities will lead tobiases in SET and questionable use of SET as an aid in instructionimprovement and determining faculty effectiveness.