Karyotypic variation in natural populations of whithefish,Coregonus lavaretus(Coregoninae, Salmonidae) from Saimaa lake system, Finland, include a remarkable and extensive length polymorphism of one arm of chromosome No. 1. This chromosomal arm (p) was partly heterochromatic and occurred in three distinct length phenotypes. These phenotypes combined in two homozygous and three heterozygous combinations; the homozygous condition for the shortest phenotype was not observed. The frequency analysis of occurrence of these phenotypes in the populations may indicate natural selection against the such shortest phenotype. It is hypothesized that the homozygous condition for this phenotype may be lethal for the carriers.