Wirth's (1938) theory of urban life has been eclipsed in recent years by a perspective that denies the importance of ecological factors. This view, though more accurate than Wirth's, fails to account for the pervasive "unconventionality" (deviance, invention, etc.) of urban life. A model is presented here to remedy that problem; it reintroduces the variable of size but in a manner distinct from Writh's. Population concentration produces a diversityof subcultures, strengthens them, and fosters diffusion among them. Together, these three mediating variables account for urban unconventionality. The four propositions of the theory and three others deducible from it are examined against existing research.