Theories that explain variations in party systems typically emphasize the role of political institutions and social cleavages. Using a panel dataset of election returns from 15 Indian states from 1967 to 2004. this article establishes considerable variation in the effective number of parties across states and over time, despite the same political institutions and relatively stable social cleavages. We argue that a hitherto ignored dimension, the level of party organization, has a significant impact on the nature of the party system. The level of party organization incentivizes politicians differently in terms of their decision to stay, join another party or float a new party, when their ambitions are thwarted within a party. To test this theory, a unique indicator of party organization is developed on the basis of extensive qualitative research. We find that in Indian states where parties are more organized, both the effective number of parties and electoral volatility are lower.