World cities attract two major streams of migrants – those who occupy the upper levels of their occupational hierarchies (the 'globalized professionals') on the one hand and marginalized, low–skill workers on the other. These two groups are often of different ethnic status, and it is argued that multicultural world cities are thus fractured in both their labour and housing markets along ethnic as well as economic lines. There has been little formal testing of these ideas, however. In this introductory article we introduce a method that can be used for comparative studies of residential fragmentation – an index of residential concentration – and apply it to three cities which have experienced rapid, multiethnic immigration in recent decades (New York, Sydney and Auckland). The patterns displayed indicate much greater fragmentation in New York than in the other two cities, suggesting that the 'conventional wisdom' regarding ethnic residential patterns may over–emphasize 'American exceptionalism'.