Thirteen chromosomal duplications, all unlinked to their linkage of origin, have been identified following X-irradiation. Ten are X-chromosome duplications, of which six are half-translocations on three autosomomal linkage groups and four are free fragments. Five of the half-translocations are homozygous fertile and two are recognizable cytologically as chromosome satellites, both of which show some mitotic instability. The free-X duplications show varying tendencies for loss. Three appear not to overlap in extent previously identified free-X duplications. The fourth carries genes from linkage group V, as well as X. Three duplications of a portion of linkage group II were identified and found to be free and quite stable in hyperploids. Some of the free duplications tend to disjoin from the X chromosome in males. New X-chromosome map data are presented.