The genetic structure based on isonymy and on gene frequencies of 7 enzyme systems was studied in a sample of 1361 individuals residing in the Ferrara Province in the Po delta (northern Italy). The sample was divided into two subsamples. The first, or indigenous, sample (n = 885) was composed of persons born and residing in the same commune of the province; the second, or migrant, sample (n = 476) was composed of persons who immigrated from a different commune. The study of the seven polymorphic genetic systems shows that there is no significant difference in gene and genotype frequencies between the two subsamples. On the other hand, the migration indicator derived from isonymy of family names is significantly larger in the migrant group than in the indigenous group. Isonymy techniques permit the detection of recent migration even under equality of gene frequencies.