Using a sample of White and Native American high school students, the authors provide a test of (a) self-control theory's invariance thesis and (b) native traditionalism as an explanation of Native American substance use. Self-control significantly influenced all forms of substance use when controlling for race and in race-specific analyses. However, "z" tests by race revealed that self-control is a stronger predictor of marijuana and serious drug use among Native Americans. Beyond this simple comparison across groups, the authors control for native traditionalism (as a proxy for cultural variation) among the Native American respondents. In doing so, self-control remained a consistent predictor of their substance use. Although these findings largely support the invariance thesis of self-control, the racial difference related to marijuana and serious drug use poses a theoretical challenge. With regard to native traditionalism, results suggest that those most attached to their native traditions engage in greater substance use. (Appended are: (1) Descriptive Statistics; and (2) Unstandardized Ordinary Least Squares Coefficients, Standard Errors, and z Scores--Self-Control on Substance Use. Contains 2 notes and 3 tables.)