At El Presidio de San Francisco, Spain's northernmost outpost in the Americas, military settlers refashioned their social identities by refusing the racializing sistema de castas and asserting a shared colonial identity as Californios. Dress was a central practice involved in this case of colonial ethnogenesis. Many studies of identity transformation in the Spanish Americas have treated dress as an individualistic medium for personal expression and social mobility. However, the artifacts and archives of El Presidio de San Francisco point instead towards the central role of colonial institutions in `fashioning' colonial subjects. While government-issue clothing functioned as a social leveler among the new Californios, dress also set firm constraints on the metamorphosis of social bodies. The findings of this study raise questions about the limits of ethnogenesis, both as a strategy used by historic subjects and as a theoretical model of identity transformation.