Mean daily milk yield for 478 Holstein-Friesian cows from two herds, recorded between 1984-1985, and for 218 Jersey cows from two herds, recorded between 1984-1985, were used to estimate lactation curve parameters for Wood’s gamma function: Y_n = A n^be^-cn.Y_n is the predicted average daily milk yield in the n^th week; A is a scalar representing ‘starting yield’; and e is the base of natural logarithm. The values of b and c (lactation curve constants describing the rates of rise to and decline from peak lactation, respectively), lactation persistency (s), goodness of fit (R²) and projected 44 week milk yield were estimated. The analysis of variance indicated significant effects (P <0.01) of herd, parity and season on most of the lactation curve parameters in both breeds. The year effects were significant (P <0.05) in the Jersey but non-significant (P >0.05) in the Holstein-Friesian. The use of monthly or weekly milk yield totals to calculate mean daily yield had highly significant effects (P <0.01) on the lactation curve parameters of the Jersey.