Guayule is an evergreen shrub that originated in the arid zone from the southwestern part of the USA to northern Mexico and therefore can be grown in environments totally different from those suited for para rubber trees (the current primary source of natural rubber (*1)). Also, the rubber constituent contained therein is very similar to that of para rubber trees. Given these factors, guayule is expected to become a new source of natural rubber. *1Hevea brasiliensis, Euphorbiaceae; evergreen tree originated in Brazil. Latex extracted from the tree contains natural rubber.