We take advantage of the fortuitous randomization of trial outcome to provide a novel strategy to identify the deterrent effect exclusive of incapacitation. We compare the post-sentencing criminal activity of criminals who were convicted of a strikeable offense with those who were tried for a strikeable offense but convicted of a nonstrikeable offense. As a robustness check, we also make this comparison in states without three-strikes laws. The identification strategy lets us estimate the deterrent effect nonparametrically using data solely from the three-strikes era. We find that California's three-strike legislation significantly reduces felony arrest rates among the class of criminals with two strikes by 17-20 percent. (Contains 3 tables, 8 figures, and 26 footnotes.)