Many cattle breeds have been subjected to high selection pressure for production traits. Consequently, population genetic structure and allelic distribution could differ in breeds under high selection pressure compared to unselected breeds. Analysis of <FONT FACE="Symbol">k</font>-casein, <FONT FACE="Symbol">a</font>S1-casein and prolactin gene frequencies was made for Argentine Creole (AC) and Argentine Holstein (AH) cattle herds. The calculated FST values measured the degree of genetic differentiation of subpopulations, depending on the variances of gene frequencies.The AC breed had considerably more variation among herds at the <FONT FACE="Symbol">a</font>S1-casein and <FONT FACE="Symbol">k</font>-casein loci. Conservation strategies should consider the entire AC population in order to maintain the genetic variability found in this native breed.
