The MN blood groups have been determined in the zones of La Vera and Las Hurdes (Cáceres). Las Hurdes, with a population which has lived isolated during several centuries, show MN frequencies similar to those of the adjacent zones, such as Salamanca. New data on La Alberca, a village of the province of Salamanca, confirm the low M frequency of this province. The present findings do not modify our previous results in 3,100 tests of the population of Spain