The long-term goal of this research is to improve the cool temperature tolerance of cotton fiber secondary wall deposition through targeted genetic engineering. It is well known that the nightly cool period (less than 28 - 22^oC or 82 - 72^oF) of the diurnal temperature cycle in temperate growing regions such as the Texas Southern High Plains adversely affects fiber yield and quality, particularly maturity, during a limited growing season. In previous research, we have established that the cotton ovule/fiber culture system established by C.A. Beasley is a valid model for the field cool temperature response of fiber development (Haigler et al., 1991). By feeding ovules with ¹⁴C-glucose and monitoring apparent rates of crystalline cellulose synthesis and respiration (CO₂ release) by previously published methods (Roberts et al., 1992), we established that some existing cultivars of Gossypium hirsutum L. have improved tolerance to cool temperatures through at least two mechanisms: (1) ability to conserve the maximum rate of cellulose synthesis at 15°C (59°F); and (2) ability to recover after rewarming from 15°C to 34°C (93°F). These studies established Paymaster HS-200 and Paymaster HS-26 as partially cool-tolerant cultivars for mechanisms (1) and (2), whereas Acala SJ-1 performed poorly for both, and transformable Coker 312 was equally improved for mechanism (2) and intermediate for mechanism (1). However, analysis of fiber weight gain after first exposure of cultured ovules/fibers to 15°C on 18 days post anthesis (DPA; 18 DPA = time of secondary wall deposition) indicates that even Acala SJ-1 has potential to adapt to cool temperatures, but only after a lag period of more than 3 days. The cultivar- specific results have been discussed more fully previously (Haigler et al., 1994).