During a 144-wk period three generations of brook trout (Salvelinus fontinalis) were continuously exposed to mean water concentrations of methylmercuric chloride (MMC) of 2.93, 0.93, 0.29, 0.09, 0.03, and less than 0.010 (control) micrograms Hg/liter. During the first 39 wk the highest concentration (2.93 micrograms Hg/liter) produced deformities and 88% mortality of first-generation adults. At an MMC concentration of 0.93 micrograms Hg/liter, second-generation trout developed deformities and all but one female died during the 108-wk exposure. No significant effects on survival, growth, or reproduction of second-generation trout were noted at any of the lower MMC concentrations. No toxic symptoms were observed in the third generation at the three lowest MMC concentrations. The maximum acceptable toxicant concentration (MATC) for brook trout exposed to MMC in this water with a hardness of 45 mg/liter (as CACO3) and a pH of 7.5 fell between 0.93 and 0.29 micrograms Hg/liter. The mean 96-h LC50 for yearling (200 g) and 20-wk-old (12 g) juvenile brook trout exposed to MMC was 75.0 micrograms Hg/liter, and the application factor (MATC/96-h LC50) lies between 0.004 and 0.013.