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Differences between daughter-dam and paternal sib estimates of heritability of first lactation milk yield have not been explained for New York data. Deviations of first lactation records from herdmate means of 155,070 artificially sired Holsteins were analyzed to examine the effect of unequal within group variances on the paternal sib estimate of heritability. The statistical model used to describe the data was yy = μ + si + eiy with σ 2yi = σ 2s + = σ2ei. Estimates of the within sire-group variances, σ1 2ei, were heterogeneous (P < .05) for each year of freshening, 1951-64. Average heritability estimates were .24, for both the usual analysis which assumes σ2ei = σ2e for all i and the method which estimates oG as σ2s as σ12s,each as σ2ei as σ12ei and heritability as 4σ12s/ σ12s + σ12ei). Similar results were obtained for other breeds. Repeatability of σ12eifrom year to year was estimated as .39 if yearly σ12ei was expressed as a fraction of the average within group variance for that year and as .58 if yearly changes in average variance were ignored. Estimates of genetic value changed little when using σ12ei in tile regression of the group mean on true daughter superiority as compared to using the pooled within group variance in the regression coefficient.