The flow of carbon in vivo from glucose to cellulose in developing cotton fibers which were actively engaged in the deposition of a cellulosic secondary cell wall was traced. Steady state levels of nucleotide sugars were analyzed from developing cotton fibers harvested between 13 and 36 days post-anthesis (DPA). The UDP-sugars were the predominant nucleotide sugars. UDP-glucose constituted over 75% of the UDP-sugar fraction, but UDP-galactose and traces of other UDP-sugars were also detected. The UDP-glucose levels increased from 0.2 ..mu..mol/boll at 13 DPA to over 2.1 ..mu..mol/boll by 24 DPA, just prior to the maximum rate of secondary wall cellulose synthesis; the levels dropped precipitously at the time when cellulose synthesis ceased. Fibers, cultured in vitro possess a very similar nucleotide-sugar composition to that of plant-grown fibers. When such fibers are pulse-labeled in vivo with (/sup 14/C)glucose, UDP-glucose is the predominant nucleotide sugar labeled. Pool size and rates of labeling of glucose, glucose-phosphate, and UDP-glucose pool data were analyzed using a computer simulation model, and it was determined that the rate of synthesis and turnover of UDP-glucose is more than sufficient to account for the combined rates of accumulation of sucrose, sterylglucosides, ..beta..-1,3-glucan, and cellulose. These data strongly indicate more » that UDP-glucose is a precursor to secondary wall cellulose in the cotton fiber. « less