Abstract : The phototoxicity of hydrogen chloride (HCl) gas was studied with particular emphasis on various external plant stresses. Greenhouse grown plants and indoor exposure chambers were utilized to test the effect of viral infection, insecticide treatment, weed killer applications, or antioxidant protectants on injury caused by HCl gas. Plant sensitivity did not differ between virus-infected and non-viral plants. Increased injury, however, was seen on plants exposed to HCl gas after one of the insecticide treatments. An experimental antioxidant compound which protects plants from ozone injury did not increase tolerance to HCl gas. Since greenhouse conditions eliminate many natural stresses, HCl exposures were conducted in field plots both in Riverside and at Vandenberg Air Force base, California. Some plants were fumigated only once while others received weekly doses. Field plants were generally more tolerant than the same species grown in the greenhouse. Further, plants exposed at Vandenberg were more sensitive than those at Riverside. Of the plants tested, a native species at Vandenberg was most resistant to HCl. HCl for the field work was generated either by diluting pressurized dry gas in a large volume of flowing carrier gas or by open-burning of small pieces of solid rocket fuel. Portable chambers remained over the field plants only for the duration of the 15-minute exposures. HF gas is considerably more phytotoxic than HCl. Work with this pollutant was limited to a review of the literature and an experiment on the uptake of fluoride salts through the roots.