Recent articles by Dear and Flusty, Logan, Marcuse and Nijmann have initiated a debate regarding the characteristics of modern and post-modern urbanism. One of the differences between the two, it is contended, is in their residential geographies: post-modern cities, it is argued, are more heterogeneous and fragmented than modern cities, which are characterized by extreme segregation of ethnic groups into citadels (the host society) and enclaves/ghettos (all other groups). Testing these ideas empirically requires a methodology which allows rigorous comparisons over space and time. This article uses a recently-developed comparative methodology to explore differences in their residential patterns between two modern (Chicago and New York) and two post-modern (Los Angeles and Miami) US cities, using data on the four main ethnic groups from the initial Census 2000 releases. It shows very clear differences between the two pairs, in the expected directions, but also differences within pairs - especially the two post-modern cities.