With the recent opening of Slow Boat Brewery in Beijing, the city's number of American-style microbreweries officially doubled -- to two. But according to both brewers, there's a growing and largely untapped market in China's capital as disposable income rises and beer-swilling residents clamor for more variety at the pub.

Late last year, Slow Boat held an evening tasting of its beers, whose flavor resembles brews of the U.S. Pacific Northwest such as Sierra Nevada. The beer ran out in just 45 minutes, despite the brewery...