This paper examines the cultural politics and commercialization of Cinco de Mayo festivals in southern California from 1930 to 1950. In the context of racial segregation and limited economic opportunities that inflicted the Mexican population, Cinco de Mayo fiestas promoted cultural pride and community solidarity. I show how Mexican Americans transformed Cinco de Mayo, over span of two decades, from a strictly nationalist celebration extolling the virtues of Mexican nationalism to a bicultural event that expressed their newfound cultural identity.  The process of cultural change and "inventedness" of ethnicity, however, was not without conflict and struggle (Hobsbawm and Ranger 1983).  The festival’s predominantly American-born and male leadership encountered tensions with Mexican nationalist groups and female organizers reflecting larger generational, ethnic, class, and gender divisions within the community. Apart from these community pressures, Mexican American fiesta organizers faced new challenges in the postwar years: Anglo city officials and Mexican government representatives intent on using Cinco de Mayo celebrations to promote “goodwill” intercultural and inter-American relations as part of the Good Neighbor Policy; companies seeking to advertise their products, sponsor queen candidates, and transform the patriotic celebration into a commercialized event.  I argue that Mexican Americans not only used Cinco de Mayo festivals to promote ethnic solidarity but as an instrument for political opposition, by appropriating the cultural pluralist discourse of corporate sponsors to seek community resources and demand full participation in the American body politic.  Mexican Americans seized upon what Mary Kay Vaughn (1994) has termed, "interactive spaces" of patriotic festivals to redefine identities and redirect energies towards community-building projects, and most of all, demonstrate to the ethnic Mexican and Anglo community that they had indeed become a political force to be reckoned with.