The objective of this study was to find measurable characters to aid the plant breeder in predicting both rubber percentage and rubber yield of guayule (Parthenium argentatum Gray) . A field of 234 guayule breeding lines was harvested by clipping in February 1986 . Forty-two of these lines were selected for further evaluation on the basis of superior regrowth, rubber content and yield . These 42 lines were reharvested in February 1987 . Nineteen variables were evaluated including percent rubber and resin, rubber and resin yield (g/plant), plant height (cm), width (cm), and volume (m3), fresh and dry weight (kg/plant), percent dry weight, stem number, mean stem diameter (cm), total and mean stem circumference (cm), total and mean stem area (cm 2), total stem circumference/total stem area (cm -1), and regeneration at two dates . Dry weight (kg/plant) had the best correlation to rubber yield (r = 0 .85), and plant volume best predicted dry weight (r = 0 .83) . These variables did not correlate as highly with percent rubber . The best statistical regression model for percent rubber included four variables, fresh and dry weight, percent dry weight, and plant volume . This model accounted for 51% of the variation . In comparison, the best regression of rubber yield had an R2 = 0 .85 for plant height and width, volume, and dry weight .