This report analyzes a large sample of U.S. grocery warehouse operators in 54 well defined grocery marketing areas. Almost all grocer retail chains with more than 40 supermarkets and $500 million in retail sales in 1990 are vertically integrated into wholesaling. More than four-fifths of the market areas display high levels of sales concentration (four-firm concentration greater than 60 percent). The 1992 merger between Super Value and Wetterau violated federal merger enforcement guidelines in at least four market areas, and several more horizontal mergers between merchant grocery wholesalers have been consummated since then.