We studied Gc, Hp, C3 and Bf serum protein polymorphisms in the population of the Middle Sangro Valley, an area in the Abruzzo Apennine mountains on the border with the Molise region, which has maintained a highly isolated character. The gene frequency estimates obtained for the total sample of the Middle Sangro Valley were: [table: see text] The analysis of these data reveals internal heterogeneity in the population of the Middle Sangro Valley probably due to geographical and socio-cultural differences which may have give rise to local micro-evolutionary phenomena.