A number of states have adopted laws that require employers to use the federal government's E-Verify program to check workers' eligibility to work legally in the United States. Using data from the Current Population Survey, this study examines whether such laws affect labor market outcomes among Mexican immigrants who are likely to be unauthorized. We find evidence that E-Verify mandates reduce average hourly earnings among likely unauthorized male Mexican immigrants while increasing labor force participation and employment among likely unauthorized female Mexican immigrants. In contrast, the mandates appear to lead to better labor market outcomes among workers likely to compete with unauthorized immigrants. Employment and earnings rise among male Mexican immigrants who are naturalized citizens in states that adopt E-Verify mandates, and earnings rise among U.S.-born Hispanic men.