The frequency of sister chromatid exchanges (SCE’s) in the euchromatin and heterochromatin of a human 9qh+ chromosome was determined using the BrdU-Giemsa technique. The 9qh+ region contained fewer exchanges than the euchromatin, and there was an excess number of exchanges at the junction of the euchromatin and heterochromatin of the long arm. BrdU incorporation and differential staining of sister chromatids showed that the heterochromatic region of the 9 was always the last region to replicate and was among the last areas of the whole human chromosome complement to complete replicatio