Two Spanish eastern Pyrenean populations, Andorra and Pallars Sobirà, have been tested for G1m(1,2,3,17), G2m(23), G3m(5,6,10,11,13,14,15,16,21,24,28) and Km(1) immunoglobulin allotypes. Km allele and Gm haplotype frequencies in both samples fit well into the Western Mediterranean and, more strictly, Pyrenean ranges with some peculiarities: Andorra showed an elevated frequency (14.7%) of the typical Asian and European Gm21,28;1,2,17;.. haplotype, while Pallars Sobirà was characterized by high values (3.7%) of Gm5*;1,17;.., a typical sub-Saharan Gm haplotype. Gm diversity assessed through genetic distance and variance analyses revealed a significant geographic partition (4.3%) of Mediterraneans among south, north-east, and north-west groups. It is interesting to note the relatively low genetic variance (2.1%) found between south and north-western Mediterraneans that could reflect ancient population relationships. More locally, genetic boundaries and diversity analyses failed to indicate any geographic pattern and/or genetic differentiation related with the political border in the Pyrenees. The present pattern of variation in this area is probably the result of genetic isolation processes, in addition to some specific demographic phenomena, in the Pyrenean valleys.