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Interaction of genotype with environment was studied with 10,780 Spanish production records (daughters of 210 Holstein-Friesian bulls) and two subsets of United States data (800,821 records with 1170 sires and 762,152 records with 1186 sires). Only 74 bulls had daughter records in both Spanish and United States data. Genetic and phenotypic (co)variances and correlations and heritability for milk and fat yields were estimated both within country and between countries with countries considered as separate traits (joint analysis). (Co)variance components were estimated with a REML procedure. Heritability estimates for milk and fat in the Spanish population (.16 and .14, within country; .12 and .09, joint) were smaller than for United States data (means of .33 and .31, within country; .26 and .24, joint). Genetic and phenotypic correlations between milk and fat within country were higher for Spain (.94 and .91) than for United States data (means of .66 and .81). Genetic correlation between countries averaged .81 for milk and .50 for fat. Rankings of bulls for milk yield are expected to be similar in Spain and the United States, although a scaling effect on predicted values is expected from different genetic variances in each country. Ranking of bulls for fat yield may be quite different.