Considerable research has documented the positive effect of teacher expressiveness and warmth on students' evaluations of college teachers, but the effect of teacher expressiveness on student performance is less clear. To investigate the interaction between teacher expressiveness, teacher sex, and student sex, 121 college students viewed a videotape of a male or female actor giving a short lecture using either expressive or nonexpressive communication. Students then completed a content test and a teacher evaluation. Results showed that the expressive teacher received the highest student evaluations using a global evaluation score as well as five factor scores. The nonexpressive male teacher, in particular, received very low ratings on two factors: organization, and stimulating interest. His students also had the poorest performance on the achievement test. In contrast, students who watched a nonexpressive female teacher had the highest achievement. Differential attention as a function of sex-role appropriate characteristics were hypothesized as being mediating variables. Male and female students reacted to the instructors in similar ways, except that female students tended to view all professors as more organized than did male students. Future research might do well to examine teacher sex in studies of teaching performance and effective teacher qualities. (Author/JAC)