The relations of luster and fiber properties have been studied in numerous specimens of a limited number of varieties, both as natural and as mercerized yarns. The differences of group averages for natural yarns of several Upland varieties appear due to yarn structure factors, but the high luster of American-Egyption cotton must be due to other factors. The gain in luster on mercerization tends to be greater, the greater the original luster, both between and within variety groups. Within one variety, Deltapine 15, the trend of the luster of mercerized yarn is to increase with increase of the fiber properties of length, wt./in., and maturity, or combinations of these. Fiber strength has no consistent relation to luster within this group. Length shows the same between and within group relations, but wt./in. shows the opposite relations between groups, and maturity shows no trend between groups. Fiber properties or combinations can be used to select for better than average luster in the longer cottons, but there is still considerable range and overlapping. The improvement possi ble by selection within a group is small compared with either mercerization or use of American- Egyption instead of Upland cotton, but suggests the worth of continued studies in this direction.