Historically, strong teacher unions have been successful at gaining benefits and security for their members, but they have been put on the defensive by recent proposals for education reform. Charter schools are one such reform that could threaten unions, but there is wide variety in the content of state charter school laws. Using state-level data from 3 different years, I find that the stronger a state’s teacher union, the more antiunion provisions a state’s charter school law will contain. These results suggest that antiunion sentiment has reached a level high enough to overcome the unions’ ability to influence policy on this issue.