The observable toxic effects produced by short-term exposure of fathead minnows (Pimephales promelas) to 2,4-dichlorophenol were reduced when the pH of the test water was increased by the addition of NaOH. After exposure for 192 h to 7.43 mg 2,4-dichlorophenol 1-1, the average survival of fathead minnows ranged from 28% at pH 7.57 to 100% at pH 9.08. Normal schooling behavior was completely disrupted, and the equilibrium of most fish was affected after a 24-h exposure to 7.43 mg 2,4-dichlorophenol 1-1 at pH 7.57, but neither schooling nor equilibrium were affected even after 192 h at pH 8.68 and 9.08. Schooling and swimming behavior of fathead minnows exposed to 12.33 mg 2,4-dichlorophenol 1-1 were affected at all pH levels. Survival of these fish after 24 h ranged from 0% at pH 7.84-46% at pH 8.81. Sodium chloride in concentrations ranging from 0 to 13.9 mg/l had no observable effects on the acute toxicity of 2,4-dichlorophenol to fathead minnows.