This dissertation evaluates the changes in food retailing patterns in the face of openmarket trends in a medium sized city (Citlalicalli) in central Mexico. This evaluation is carried out using the model of retail trade-concentration. The following questions are addressed: whether trade-concentration had occurred in the field-site; if yes, what were the causes; and, what were the effects of retail concentration on preexisting food retailers. The fieldwork for this study was conducted over a 14 month period (May 2005 and June 2006). Empirical data were collected using three surveys, over 25 interviews, and several hours of participant observations. Along with these primary data, secondary data from official records were also collected. The survey data were analyzed using nonparametric tests. The findings from the study revealed that trade-concentration is proceeding in Citlalicalli and the recent spread of large stores in the region has been encouraged by free-trade policies adopted by Mexico since the mid-1980s. Trade-concentration in the Altepetl metropolitan area has also been aided by the positive behavior that consumers show towards large retail chains. Contrary to popular belief, the impact of retail concentration on preexisting small retailers in Citlalicalli has been minimal. Partly traditions and partly the nearness of a particular store explain the continued patronization of small stores by consumers. The product category and income of consumers also influence their purchase decisions. This research shows that competition in food retailing in developing countries is between same size and same type retailers. Small retailers are not always in direct competition with modern retailers as they serve different sections of the society. Retail development is a part of urban development and, therefore, has direct implications for the quality of life in cities. In emerging economies, cities like Citlalicalli have vibrant town centers which provide convenient access to offices, shops, and entertainment areas for its citizens. This study concludes that appropriate policy measures can ensure the coexistence of large and small retailers in developing countries and, thereby, the continued vitality of city centers.