The timing of late replicating bands on the X chromosome has been studied in individuals with the fragile X [fra(X)] syndrome. Compared to controls both affected individuals and symptomless carriers of the syndrome show delayed replication of the Xq27 region as shown by 2 different methods. The implications of this finding are discussed in relation to the proposal [Laird et al., 1987] that the fraX syndrome is associated with a failure to reactivate the Xq27 band correctly after it has been inactivated in a female.