Mercury toxicity in rainbow trout fingerlings was related to temperature and chemical formulation of the mercury. At 10 C, the 24-hr TLm concentration (median tolerance limit) for mercuric chloride (HgCl2) was approximately 30 times that for phenyl mercuric acetate (PMA). The 96-hr TLm values for mercuric chloride at 5, 10, and 20 C were 0.40, 0.28, and 0.22 mg Hg/liter. The velocity of mortality, (V = reciprocal of time to death in hours), was linearly related to temperature. For a mercuric chloride concentration of 0.5 mg Hg/liter, V =.002 +.0023t, where t = temperature in centigrade degrees.Temperature also affected accumulation rate of mercury in the fish muscle. At 5, 10, and 20 C a mercuric chloride concentration of 0.1 mg Hg/liter in the water produced biological magnification factors (conc. in fish ÷ conc. in water) of 4, 10, and 22 ×, respectively, PMA produced higher magnification factors of 80–100 × at 10 C.Active metabolic rate, though increased by higher temperatures, was depressed by mercuric chloride, and higher temperatures augmented the depressant effect.