In a Finnish rural population 79.0 % of 233 persons had the cusp of Carabelli in first upper molars. Of these 20 % showed |DA positive cusps |DA. and 59 % |DA negative cusps |DA. The occurrence of the structure was bilateral with varying degrees of asymmetry, but if there was no structure on one side of the jaw, the other never showed the cusp. There was no sexual dimorphism either in the occurrence or in the degree of expression of the character. Results of the statistical analyses suggested low heritability of the character. However, the dichotomy of having a cusp or not may have genetic basis, but there is large variation in the expression of the |DA cusp-genotype |DA.