Gabbro is a mafic rock with many species that do not occur on soils of granitic or ultramafic rocks. Although some gabbro soils harbor many unique or endemic species, others do not, as botanists have noted from the Appalachian Piedmont to the mountains of California and Oregon. Gabbro soils were sampled in the Peninsular Ranges and in the foothills of the Sierra Nevada to identify special features of the gabbro soils with unique plant species distributions. No soil morphological or chemical differences were found between gabbro soils with and without unique plant species that might explain the differences in plant distributions. Although many unique species may occur only on gabbro soils, apparently their distributions cannot be explained primarily by soil differences among gabbro soils.