Allele frequencies at the phosphoglucomutase-1 (PGM1) locus have been investigated in two Croatian (Yugoslavian) populations from neighboring islands, Silba and Olib. The genotype distributions are significantly different though the two islands are only 2 km apart. In the light of demographic and historical data, a few hypotheses explaining these results are discussed. A rare variant, PGM1*W3, usually found in Asia, is present in 4 inhabitants from the Olib island.