The genetic structure of natural populations of the domestic cat was examined at the microgeographic level (in the Spanish city of Barcelona) and the macrogeographic level (in Catalonia in Spain, and in upper midwestern USA) using frequency data for seven monogenic morphological traits. At the microgeographic level in the city of Barcelona there was no evidence for nonrandom mating within colonies, and estimates of between-colony gene flow were quite high. At the macrogeographic level, the populations from Catalonia and upper midwestern USA differed in two major respects: (i) The Catalan populations were in reasonably good agreement with expectations of Hardy-Weinberg equilibrium while the North American populations showed some evidence of the Wahlund effect (overall heterozygote deficiency indicating population substructuring). (ii) In the Catalan populations, approximately fifty per cent of the genetic differentiation between populations could be explained by geographical separation while in North America only four per cent of the total differentiation was attributable to geographical distance.