The general theory of crime (Gottfredson and Hirschi 1990) proposes that self-control is the primary individual-level cause of crime and that its effect is contingent on criminal opportunity. This study conducted a prospective test of self-control and opportunity as predictors of property crime and personal crime among drug-using offenders. Each predictor had a main effect; property crimes and personal crimes were more frequent among offenders lower on self-control and those with higher opportunity. A significant interaction between these predictors was also detected. About four percent of the variance in each type of crime was explained by these predictors. Results support the proposition that self-control is a causal factor in criminal behavior and suggest that its effect is partially contingent on opportunity, but self-control and opportunity, as measured here, had very modest explanatory power.