In the United States, a majority of the drivers who receive a traffic ticket are male, and male drivers are more likely to receive a ticket after being stopped by the police. This paper develops and conducts an empirical test for the existence of police gender bias (taste-based discrimination) in traffic ticketing. The test is based on a model's prediction of how the gender composition of ticketed drivers should vary across groups of police officers who use unbiased, but potentially different ticketing standards. The test is useful for determining whether the gender disparity in traffic tickets results from gender bias or a higher tendency of male drivers to break traffic laws. In addition, the test offers an improvement over the "differences-in-differences" test for discrimination which has been applied in other contexts. When applied to data on traffic tickets issued by male and female police officers in Boston, the new test rejects the null hypothesis of unbiased ticketing.