Technical Abstract: RMBUP-C4 (Random Mated Barbadense Upland Population Cycle 4) (Reg. No. GP_____: PI______) is a unique random mated germplasm population of Upland cotton (Gossypium hirsutum L.) which has introgression of G. barbadense L. alleles. This population involved five cycles of random mating beginning with 53 top crossed F1 lines. The germplasm was developed through cooperative research by USDA, ARS, Mississippi Agricultural and Forestry Experiment Station, and Cotton Incorporated. Cultivars ‘Sure-Grow 747’, ‘Phytogen 355’, and ‘Fibermax 966’ were each crossed with 18 CS-B chromosome substitution lines; however, the seed of Phytogen 355 x CS-B22sh was lost. The bulked pollen method of pollination was used in the development, and there were five cycles of random mating with the intercrossing of F1 lines considered as cycle zero. After randomly mating the 53 F1 lines, the cultivars lineage within each CS-B line were combined which produced 18 individual populations. The 18 populations were planted as 18 individual populations and randomly mated among populations each additional cycle. Because the CS-B lines were each euploid chromosome substitution lines with entire chromosomes or arms from G. barbadense substituted into Upland, we should have achieved considerable introgression of alleles from G. barbadense into this randomly mated population. This population should be of value to cotton breeders and geneticists across the U. S. Cotton Belt as a unique population with G. barbadense introgression.