A rare variant of blood group A, A(finn), is used as a marker when studying the Finnish population structure. Slow immigration is probably the main cause of the primary enrichment of this gene that has not yet been described outside Finland.Practically all the 98 propositi originated from the areas in the south-west and south-east that were first settled in prehistoric times. This indicates that both were inhabited by the same settlers, probably immigrating over the Gulf of Finland rather than using two different routes. The frequency of A(finn) in the first settled areas was estimated at approximately 1:1,000, showing large local differences at community level, with estimated peaks as high as more than 1%. The frequency distribution was analysed in a computer simulation programme; the similarities with the observed distribution of A(finn) were compatible with the effect of sampling and drift, causing substantial loss and large frequency deviations in the isolated subpopulations.