The company says it sold 489,616 Focus sedans and hatchbacks globally in the first half of 2012, besting the Corolla by almost 27,000.
The Focus has several advantages, said Aaron Bragman, an automotive industry analyst for IHS Global Insight. It is newer, features better handling and styling, has superior technology and a much nicer interior than the Corolla.
"I'd rank it superior to the Corolla in pretty much most ways," Bragman said.
But Toyota isn't conceding the title. It has disputed Ford's numbers, saying its car is still on top when you include Corollas sold under other names.
Hatchback versions of the Corolla are sold as the Auris in Europe and Matrix in the United States.
Toyota also sells the Corolla Verso, a van-like vehicle that the company has traditionally counted as a Corolla, spokesman Mike Michels said. Including all the models, Toyota sold 603,840 Corollas in the first half, beating the Focus by about 114,000, Michels said.
Toyota is working on a new version of the Corolla that is expected to go on sale next year, although the company hasn't made it official.
Ford made the announcement at its assembly plant in Rayong, which opened in May to build the Focus. The Thailand plant, which is one of Ford's most advanced, has the capacity to produce 150,000 cars per year for sale in Thailand, Vietnam, Australia and other countries in the region.
But the Corolla is still king in the United States, where it outsold the Focus by nearly 27,500 in the first half of the year.