Frequency of yarn breakage and spinning limits of open-end rotor yarns produced from five Egyptian commercial cottons and one Upland cotton differing in fiber fineness, strength and length were investigated. A series of different four progressively finer yarn counts were produced from the same cotton at twist multi­plier constant (4.8). The obtained results revealed that : For the long staple Egyptian cottons. a minimum of 90 fibers in the open-end yarn cross-section is needed in rotor spinning. These numbers, . however, is not constant, but it varied according to modern spinning units, and processing conditions. The Egyptian cotton breeder should pay more attention to produce a new fine, strong and short staple cotton cultivar's to be capable for spinning higher quality open-end fine yarn counts at acceptable level of spinning performance. The Egyptian commercial cotton varieties exhibited higher spinning performance and yarn strength (lea count strength product) than the Upland MS cotton due to their superior fiber quality es­pecially fiber strength.