Improvement of fiber properties is necessary for Upland cotton, Gossypium hirsutum L., to remain competitive in today's global market. The objective of this study was to develop Sea Island/Upland (SIUP) biotypes with improved fiber length and strength. 'NMSI 1331' a Sea Island (G. barbadense L.) breeding line was crossed with Upland germplasm to develop SIUP biotypes. Individual plants were selected from the F3 generation grown in a spaced nursery. Plants were selected based on apparent yield and fiber quality potential. Eight F3:4 progeny rows were selected in 2003 and evaluated in performance trials at College Station, TX during 2004, 2006, and 2007. These interspecific, introgressed lines ranged in lint production from 618 kg/ha-1 to 979 kg/ha-1 and exhibited fiber properties equal to or exceeding those of 'Fiber Max 832' (PI 603955) and 'Deltapine 491' (PI 618609). Introgression of interspecific alleles is thought to have manifested unique fiber qualities with acceptable agronomic performance in this group of SIUP breeding lines.