The heritability of milk yield and fat percentage was calculated of herd-registered cattle in Friesland. The estimates were based on daughter-dam comparisons. Comparisons covered the first three lactations. The average heritability estimates of milk yield within sires, and according to the different kinds of soil, were 35.50 and 37.38 %, with daughter-dam regression and dam-daughter correlation, respectively. Under the same conditions the figures for fat percentage were 76.52 and 81.58 %, respectively.When the effect of different soils was ignored, average heritability estimates for milk were 36.58 and 38.60 %, respectively. The corresponding figures for fat percentage were 76.40 and 81.60 %, respectively.If the figures were not calculated on within-sire basis, the average heritability for milk yield was 39.60 and 40.60 %; whereas for fat percentage the estimates were 84.44 and 83.52 %, as calculated from daughter-dam regression, and damdaughter correlation, respectively.The most reliable estimates of heritability were by the within-sire daughterdam regression calculated for each of the different soil types.