The white button mushroom (agaricus bisporus) accounts for 90 percent of all the mushrooms grown in the United States (trend down from 95% a few years ago). Specialty varieties, including the shiitake and the oyster mushroom, make up the remainder. Sales of specialty mushrooms almost doubled between 1991-92 and 1993-94 and have grown fivefold in the past 30 years. Pennsylvania and California dominate U.S. mushroom production. Pennsylvania's production accounted for 47 percent of U.S. output in 1993-94, and California accounted for 17 percent.

The majority of mushroom farms are family operations. Many of Pennsylvania's farms have been in business three to four generations, with second-generation farms considered new. These old operations cultivate mostly white button mushrooms and have tremendous sunk costs in their buildings and equipment, making it difficult for them to respond effectively to new market entrants. Like an aircraft carrier at sea, it will take a long time for these behemoths to modify their systems, if they are interested in the first place. Moving operations (massive greenhouses and concrete bunkers, etc.) to take advantage of co-location is impractical at best. Attracting businesses to co-locate with existing cultivation operations presents interesting possibilities.

Mushroom production is dominated by several large firms, including Campbell's Soup Company, which owns eight mushroom farms throughout the U.S., and Monterey Farms, which owns five farms in California, Texas, and Tennessee. There are 5 or 6 mid-sized farms producing 5 million to 20 million pounds annually, and about 100 farms each producing 1 million to 5 million pounds annually. U.S. specialty mushroom production is dominated by six farms. These six growers account for 50 percent of US production.