The 96 h and 192 h LC50s were determined for phenol and 11 substituted phenols. The freshwater flow-through tests with measured concentrations and nearly identical test conditions used the fathead minnow Pimephales promelas as the test organism. The fish were checked for uniformity in toxicity response using 48 h static reference tests with 2,4-dichlorophenol as the toxicant. The results of these 12 tests produced a mean 48 h LC50 of 8.6 mg/L plus or minus 1.1 mg/L. The mean of duplicate 96 h LC50s (mg/L) in ascending order of toxicity were: pentachlorophenol, 0.22; 4,6-dinitro-o-cresol, 2.0; 2,4,6-tribromophenol, 6.6; 2,4-dichlorophenol, 8.2; 2-chlorophenol, 12; 2,4-dinitrophenol, 17; 2,4-dimethylphenol, 17; 2,6-dimethylphenol, greater than 27; phenol, 29; 4-nitrophenol, 61; and 3-methoxyphenol, 76.