A study of sex chromosome replication in euploid bovine cell cultures is presented. Both X chromosomes in the female replicate their DNA throughout most of the cell’s DNA synthetic period. However, one X (late X) continues its replication later than its homologue (early X). This late X is one of the last chromosomes in the complement to terminate replication. In the male, the DNA replication pattern of the X chromosome is like that of the early X in the female, while the Y chromosome begins its replication later than any chromosome in the complement (by two or more hours), and finishes replication just after the