Estimates of dog breed differentiation at microsatellite loci were obtained: (1) by contrasting data from German shepherds with a composite breed mixture sample; (2) by examining the Wahlund effect in new and published breed mixture data sets; and (3) from a comparison of published retriever and dachshund allele frequencies. In terms of F-statistics, the estimates ranged FBT = 0.22-0.30, and were therefore considerably greater than a previous estimate from allozyme data (FBT = 0.10). The new estimates are consistent with a view that the genetic basis of an average breed effectively corresponds to four genomes sampled from the total, or ancestral, dog gene pool.