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Guayule (Parthenium argentatum Gray) is a perennial shrub native to the Chihuahuan Desert of Northern Mexico and the Big Bend area of southwest Texas. One of the most valuable products from guayule is its hypoallergenic latex. However, little research has been done on the optimum harvest time for latex concentration and yield. The objective of this study was to determine the optimum harvest time during the growth cycle for latex content, plant biomass, and latex yield of guayule. Treatments consisted of three guayule lines (11591. AZ3, and G7-11TC) harvested every other month for 2 years. Plants were transplanted on 4 April 1995 at the University of Arizona Maricopa Agricultural Center, Maricopa, Arizona. Harvesting began in March 1998 and continued every other month through January 2000. Samples were analyzed for latex concentration and total biomass. Latex yields were calculated as the product of the latex concentration times the dry plant biomass. The experimental design was a randomized complete block with four replications. Results varied among lines and harvest dates. There appears to be enough differences among lines that planting lines selected for different optimum harvest dates would allow growers to spread the optimum harvest time throughout most of the year. This would also benefit processors by allowing them to reduce their production costs by spreading the harvest over several months instead of only a few months. More research must be done to determine whether specific environmental factors can be associated with the optimum harvest time and line specific harvesting guides such as growing degree days or other methods for determining optimum harvest date can be utilized in guayule. Published by Elsevier B.V.