Electrophoretic studies on Carduus pycnocephalus L., C. tenuiflorus Curt., and their hybrids, showed that, although hybridization is common, the two parental species are quite well separated by their enzymatic patterns. This was confirmed by data on fertilization and achene abortion rates of the internediate forms between the two species. The seed set of hybrids was the lowest in California, where the two species have been introduced.