Photochemical oxidants (smog) are causing serious injury to ponderosa pine in the San Bernardino Mountains of southern California. Apparently, smog injury also renders the trees more susceptible to attack by two species of destructive forest insects, the western pine beetle, Dendroctonus brevicomis, and the mountain pine beetle, D. ponderosae. Recent studies have shown that photochemical oxidant injury to ponderosa pine results in reduced oleoresin yield, rate of flow and exudation pressure, sapwood and phloem moisture content and phloem thickness, all of which are believed important in the defense of the tree against bark beetles. Smog injury also affects growth rate and probably wood quality. Soluble sugars and reserve polysaccharides were reduced in diseased trees. Current studies indicate that similar injuries to ponderosa pine, with resulting increase in bark beetle attack, occur as a result of infection by root disease fungi, notably Fomes annosus and Verticicladiella wagenerii.