How do students enrolled in online courses perform relative to those who choose a more traditional classroom environment? What student characteristics help explain differences in student academic achievement in the two modes of instruction? What factors affect the students' choice of instruction mode? The authors address these questions in relation to the teaching of introductory economics courses. They find that the two groups of students are significantly different in age, gender composition, marital status and number of children, GPA, previous economics exposure, planned major, and other important characteristics. The raw data suggested a higher mean score for the online class sections. But after considering course selection bias, the findings indicated that age and GPA positively affect students' performance in the course, whereas the online teaching mode has a narrowly insignificant, or even negative, effect. Semester effects are most important for the online subsample, and male students enjoy a premium in the traditional classroom setting. (Contains 7 tables and 19 notes.)