Technical Abstract: Commercialization of guayule (Parthenium argentatum Gray) as a source of rubber is receiving world-wide attention as an alternative to Hevea in order to meet increasing demand for natural rubber. As more breeders, agronomists, botanists, and other scientists become involved in evaluating and developing guayule germplasm, it is imperative that a uniform set of germplasm descriptors is available. These descriptors are also necessary for use in obtaining plant variety protection certificates. The objective of the current study was to develop a set of descriptors for evaluating guayule germplasm in breeding and agronomic studies as well as identify a minimum set of descriptors to use when time or labor make using all of the descriptors not feasible. Based on the experience of the authors and previous research, a set of descriptors was developed and used to evaluate breeding nurseries of both traditionally developed and transgenically developed guayule plants. The descriptors were easy to use and required a minimum amount of time per plant, so that a large number of lines could easily be evaluated. The descriptors adequately covered the range of diversity observed in the nurseries evaluated. A set of 14 minimum descriptors is proposed – plant height, plant width, biomass, number of main branches, bark thickness, leaf shape, leaf serration number, dormancy break, flower bud score, 1,000 seed weight, rubber content and yield, and resin content and yield. For germplasm protection uses, the full set of descriptors should be used. The descriptors will provide uniformity in comparing germplasm performance across environments. The descriptors will also be valuable to regulatory agencies in granting various plant protection certificates.