Four species of the genus Parthenium were involved in a hybridization attempt. The species were: guayule (Parthenium argentatum Gray), Parthenium fruticosum Less, Parthenium bipinnatifidum (Ortega) Rollins, and Parthenium incanum H. B. K. Primary attention was given to the formation of hybrids between P. argentatum and P. fruticosum. Characteristics of the latter species such as size, growth rate, possible disease resistance, and wider geographical adaptation were desirable for transfer to P. argentatum. Reciprocal crosses were made between these two species and the production of hybrids was not difficult. However, in the case of selfing, backcross and sibcrosses, germination percent was 0.86 from a total of 3,471 achenes. Therefore, a search for the cause or causes of the negative results was initiated. The factors which were considered of primary interest were: incompatibility, genic and chromosomal sterility, pollination and planting techniques, and seed germination treatment. Incompatibility was considered the main limiting factor in the formation of a backcross population. Rubber analysis was performed in the interspecific hybrids. Mean rubber percent for hybrids between P. artentatum and P. fruticosum was 1.19; and for the reciprocal cross it was 0.39. Hybrids of the cross P. fruticosum x P. bipinnatifidum had a mean rubber percent of 0.19.