A new cytoplasm male sterile line and accompanying restorer line based on the Gossypium trilobum (D8) cytoplasm has been developed. All observations were taken at Fayetteville, AR, so the influence of other environments have not been examined. During backcrosses 4 and 5 of a 6X hybrid 2 (AD(1) x D(8)) to Upland cotton, male sterile (D8ms) and male fertile (D8mf) progeny resulted. Sibling crosses (DSMS x D8mf) give all fertile progeny, whereas standard Upland cultivars as pollen donors on D8ms gave all sterile progeny. Self-pollination of D8mf gave all fertile progeny and this progeny restored fertility to D8ms lines. The S(1) of (D8ms x D8mf) had less than 2% ms and the S(1) gave 0% ms. When the S(1) of D8mf was crossed with a standard cultivar, the progeny segregated ms and mf. One model that explains the observations is that sterility is expressed at the microspore level. That is, only microspores carrying Rf will survive, whereas normal segregation of Rf and rf occurs in the megaspores.