The free competition, flow of information, and open entry of public produce marketplaces should enforce maximal economic efficiency of firms. Economic anthropologists have insisted for years that regular, customary relations between economic actors are as important as efficiency. Custom and efficiency in the economic behavior of family‐firm produce merchants in an urban, public produce marketplace in St. Louis, Missouri, are analyzed using multiple regressions. Factors representing economic custom are strongly significant in explaining gross sales. The economic anthropological position is supported using econometric data drawn from a contemporary capitalist marketplace. [economics, markets, decision making, quantitative methods, urban United States]