Samples of oleoresin were collected from natural populations of pines on Guadalupe and Cedros islands, Baja California. The monoterpenes of the individual trees were analysed by gas-liquid chromatography and compared with those of Pinus radiate and P. muricata as found in nature and in cultivation. In all the samples from the two islands the main constituents were [alpha]-pinene and [beta]-pinene. There was wide tree-to-tree variation in the proportions of these, and a strong over-all similarity to the monoterpenes found in the three natural populations of P. radiate on the mainland of California. On average, [beta]-pinene was ;75% of the total for Guadalupe Island and 80% for Cedros Island. From the taxonomic point of view, these results support (1) the inclusion of the Guadalupe Island population in P. radiate; (2) the recent transfer of the Cedros Island populations from P. muricata to P. radiate, with the new combination P. radiate var. cedrosensis (Howell) Axelrod.