The average casein micelle size varies widely between milk samples of individual cows. The factors that cause this variation in size are not known but could provide more insight into casein micelle structure and into the physiology of casein micelle formation. The objective of this research was therefore to determine factors that influence average casein micelle size in milk from individual cows. Average casein micelle size of milk samples was associated with the A and B genetic variants of k-casein, and differences in concentration of glycosylated k-casein as a fraction of total milk protein. Milk samples with a low average casein micelle size were associated with the B variant of k-casein and a higher relative concentration of glycosylated k-casein, compared with milk samples with a high average casein micelle size. Differences observed may be attributed to the effect of glycosylated k-casein groups on casein micelle formation in the mammary gland.