Molecular weight distribution (MWD) of guayule rubber extracted from well characterized, high rubber yielding plants, grown under controlled and natural conditions were observed to be multimodal (especially in young plants). The MWD curves apparently depend on genetic and environmental inter-relations, which have not been established. We believe that multimodal curves may reflect a batch type polymerization process which could be triggered by rubber-synthesizing enzyme activities, depending on genetic and environmental factors.