Soils, timber site data, and the foliage of mature conifer trees were sampled at 16 Klamath Mountains sites with serpentine soils and four sites with gabbro soils. The basal areas of trees at the gabbro sites were greater than the basal areas at the serpentine sites. Tree height growth, based on old growth site curves, was significantly greater on the gabbro soils than on the serpentine soils. The main soil difference was about five times greater exchangeable Mg from the serpentine soils than from the gabbro soils. The higher Mg in serpentine soils was not reflected in higher concentrations of Mg in the foliage of yellow pine (Pinus spp.), Douglas-fir (Pseudotsuga menziesii [Mirb.] Franco), or incense cedar (Calocedrus decurrens [Torr.] Florins) trees on serpentine soils, but Ca was significantly higher in the Douglas-fir tree foliage on gabbro soils. The high Mg in the serpentine soils may interfere with the utilization of Ca by Douglas-fir trees causing their foliage to have lower Ca concentrations than in the foliage of Douglas-fir trees on gabbro soils. For each of the three tree species (yellow pine, Douglas-fir, and incense cedar), foliar N, P, and K concentrations were not appreciably different on serpentine versus gabbro soils. Although soil N and P were not determined, organic matter concentrations did not differ between serpentine and gabbro soils.