This research examines the relative importance of attitudes and group norms in determining frequency of marijuana use. In this examination, we employ a neglected concept in attitude/behavior research—behavioral commitment. We suggest that the role of attitudes and group norms will vary, in part, depending upon one's level of commitment to the behavior. Specifically, we assert that norms will increase in importance over attitudes as behavioral commitment increases. To test this hypothesis, we utilize covariance structure analysis with a nationally representative sample of high school seniors defined in terms of their commitment to marijuana use. Multiple group comparisons also are performed to assess the comparability of measurement properties and structural estimates. Results are supportive of the hypothesis. The findings are discussed in terms of previous attitude/behavior research and drug abuse prevention programs.