The isonymy structure of Buenos Aires was studied based on its surname frequency. Information on 2,552,359 voters of the 28 Buenos Aires districts was used to estimate Lasker's coefficient of relationship by isonymy (R_i), surname diversity according to Fisher's a, the coefficient of consanguinity resulting from random isonymy (F), and Nei's, Lasker's and the Euclidean isonymy distances. These distances were correlated with geographic distances, which were calculated by assigning an arbitrary point to each district and measuring distances on a map of the city. The R_i × 10⁵ and F × 10⁴ averages of the districts located south of Rivadavia Avenue were higher (R_i = 66.08; F = 3.4) than those situated north of that avenue (R_i = 46.60; F = 2.4) (p < 0.001). Fisher's a showed the opposite behavior (north, α = 1,055.5; south, α = 757.2). There was a significant correlation (p < 0.001) between geographic distance and Nei's and the Euclidean distances (0.496 and 0.503, respectively), but the correlation was not significant for Lasker's distance (0.051). These results indicate isolation by distance in the city of Buenos Aires and reveal subdivision of the metropolitan population, with greater consanguinity and a lesser variety of surnames in the districts located in the southern section of the city. This structure agrees with the fragmentation and social, cultural, and economic differences observed among the districts of this Latin American metropolis.