Acute toxicity tests were conducted to determine (a) the individual and interactive effects of water temperature (7, 12, 17 degree C), pH (6.5, 7.5, 8.5, 9.5), and time on the toxicity of terbufos, trichlorfon, 4- nitrophenol, and 2,4-dinitrophenol to rainbow trout (Oncorhynchus mykiss) and the amphipod Gammarus pseudolimnaeus, and (b) the individual and interactive effects of water temperature and pH on chemical bioconcentration during acute tests with rainbow trout and Gammarus exposed to terbufos, 4-nitrophenol, and 2,4-dinitrophenol. The toxicity of all four chemicals was significantly affected by pH in all tests, except for Gammarus exposed to terbufos. The toxicity of terbufos to rainbow trout and Gammarus was less at pH 7.5 than at higher or lower pH. The toxicity of both nitrophenols decreased as pH increased, whereas the toxicity of trichlorfon increased with pH. The effect of pH on trichlorfon toxicity decreased with temperature. Temperature significantly affected the toxicity of all four chemicals to both species. Toxicity increased with temperature in all tests, except for rainbow trout exposed to nitrophenols; toxicity decreased as temperature increased for rainbow trout. Chemical bioconcentration was also significantly affected by temperature and pH and was directly related to toxicity in most tests. Significant interactive effects between toxicity-modifying factors were also frequently observed. Temperature and pH effects on chemical toxicity need to be considered in chemical hazard assessment to ensure adequate protection of aquatic organisms.