Three hybridizing populations involving Hulsea heterochroma A. Gray and H. vestita Gray ssp. callicarpha (Hall) Wilken were discovered in the San Jacinto Mountains, Riverside Co, California. Population structure was investigated by means of morphological analyses, flavonoid chemistry, meiotic chromosome behavior and pollen fertility. Synthetic F<sub>1</sub> and BC<sub>1</sub> hybrids were employed in assessing the degree of hybridization in the three populalations. F<sub>1</sub> hybrids were found to be heterozygous for a single reciprocal translocation and their pollen fertility was relatively high. Synthetic backcross hybrids displayed normal meiosis and high pollen fertility. On the basis of correlative studies, the hybridizing populations were found to comprise plants of both parental taxa, F<sub>1</sub> hybrids and backcross hybrids with H. vestita ssp. callicarpha as the recurrent parent. The inability to readily distinguish most putative backcross hybrids from plants of the recurrent parent in natural populations indicates that introgression involving the two taxa should prove difficult to substantiate by traditional techniques.