Do primaries help political parties perform better in general elections, or do they undermine electoral performance by contributing to internal divisions and to the weakening of party organizations? This article examines the effect of holding a primary on the general election prospects of candidates, using cases from two of the three major parties in Mexico's 2006 national legislative elections. In both parties, primaries fail to systematically produce candidates with advantages in the general election, due largely to organizational deficits of the parties and low entry requirements for aspiring precandidates. Indeed, outside urban centers, where parties tend to be better organized, primaries actually seem to hurt party performance in subsequent general elections.