Relative concentrations of alpha(S1)-casein and alpha(S2)-casein (alpha(S1)-CN and alpha(S2)-CN) phosphorylation isoforms vary considerably among milk of individual cows. We estimated heritabilities for alpha(S2)-CN phosphorylation isoforms, determined by capillary zone electrophoresis from 1,857 morning milk samples, and genetic correlations among alpha(S2)-CN phosphorylation isoforms in Dutch Holstein Friesian. To investigate if phosphorylation of alpha(S1)-CN and alpha(S2)-CN are due to the same genetic mechanism, we also estimated genetic correlations between alpha(S1)-CN and alpha(S2)-CN phosphorylation isoforms as well as the genetic correlations between the phosphorylation degrees (PD) of alpha(S1)-CN and alpha(S2)-CN defined as the proportion of isoforms with higher degrees of phosphorylation in total alpha(S1)-CN and alpha(S2)-CN, respectively. The intra-herd heritabilities for the relative concentrations of alpha(S2)-CN phosphorylation isoforms were high and ranged from 0.54 for alpha(S2)-CN-10P to 0.89 for alpha(S2)-CN-12P. Furthermore, the high intra-herd heritabilities of alpha(S1)-CN PD and alpha(S2)-CN PD imply a strong genetic control of the phosphorylation process, which is independent of casein production. The genetic correlations between alpha(S2)-CN phosphorylation isoforms are positive and moderate to high (0.33-0.90). Furthermore, the strong positive genetic correlation (0.94) between alpha(S1)-CN PD and alpha(S2)-CN PD suggests that the phosphorylation processes of alpha(S1)-CN and alpha(S2)-CN are related. This study shows the possibility of breeding for specific alpha(S1)-CN and alpha(S2)-CN phosphorylation isoforms, and relations between the phosphorylation degrees of alpha(S1)-CN and alpha(S2)-CN and technological properties of milk need to be further investigated to identify potential benefits for the dairy industry.