Morphological and isoenzyme variation among populations of western azalea, Rhododendron occidentale (Torr. & A. Gray) A. Gray, were examined. Three regional parapatric groups were revealed: 1) the northern California outer North Coast Ranges; 2) the northern California and southern Oregon Klamath Ranges; and 3) the central California Sierra Nevada and southern California Peninsular Ranges. A highly variable but generally intermediate fourth group is restricted to ultrabasic substrates (serpentine) in the middle and inner North Coast Ranges of California. It is comprised of populations with recombined morphologies and alleles that were otherwise restricted to one or more of the three groups above. A revised intraspecific treatment is proposed, with the three regional groups above recognized as varieties. These are: R. occidentale (Torr. & A. Gray) A. Gray var. occidentale (outer North Coast Ranges), R. o. var. paludosum Jeps. (Klamath Ranges), and Rhododendron. occidentale var. californicum (Torr. & A. Gray) Hrusa comb. et stat. nov. (Sierra Nevada and Peninsular Ranges). Lectotypifications of Azalea occidentalis Torr. & A. Gray isotypes at PH and GH, Azalea californica Torr. & A. Gray in Durand, and Rhododendron sonomense Greene (NDG) are also provided.