The Y chromosome of the Chinese hamster is late replicating in spermatogonial mitoses labelled in vivo. During the last two hours of the S phase the long arm of the Y replicates the majority of its DNA. The short arm terminates replication before this. Replication of the long arm of the X takes place earlier in S, whereas the short arm of the X is synchronous with that of the Y. Two small metacentric autosomes are also late replicating, in synchrony with the long arm of the Y. The distinct pattern of late replication of the Y chromosome of spermatogonia is in agreement with our experience with other mammals. The replication patterns of spermatogonia and a morphological study of the XY bivalent in primary spermatocytes indicate that the X and Y chromosomes pair by their short arms.