Houston won't know who its new mayor will be until next month.

City controller Annise Parker and former city attorney Gene Locke are headed to a runoff to become mayor of America's fourth-biggest city.

Parker, who would be the first openly gay mayor of Houston, collected nearly 31 percent of the vote Tuesday.

Locke, with 25 percent, topped architect and urban planner Peter Brown, who had nearly 23 percent.

A runoff is needed because no one received 50 percent of the vote. A specific date has not yet been set for the election, but it will be in December.

A victory for Parker would make Houston the largest U.S. city with an openly gay mayor.