It has been known for decades that "growth rings" are revealed by swelling cotton fibers that have developed on plants exposed to night temperatures below about 27 C, but little is known about the cellular cause of this phenomenon. Fibers that develop in this way are immature and do not command a premium price for the grower or provide optimal spinning properties for the processor. We have demonstrated that "growth rings" are also observed in fibers developed from dissected and cultured ovules (according to the method of Beasley) that are exposed to cycling temperatures in the incubator (18 hrs., 34 C; 6 hrs., 20 C) during the period of secondary wall deposition. The rings are not observed in control in vitro fibers incubated continuously at 34 C. This demonstrates that "growth rings" result from an effect of cool temperatures an a specific temperature-sensitive step in cell wall deposition; they do not result from an effect an whole-plant physiology. Preliminary results from incorporation of radioactive glucose into cultured ovules incubated at 34 C, 28 C, and 20 C indicate that the rate of synthesis of crystalline cellulose decreases linearly with temperature. The ability to induce "growth rings" in cultured fibers will facilitate eludication of their cellular cause. This work was supported by Cotton Incorporated and the Texas Advanced Technology Program.