Fathead minnows Pimephales promelas were exposed for 32 days, starting with embryos and continuing through early juvenile development, to five concentrations of 1,2,4-trichlorobenzene (1,2,4-TCB) with a control. Each series of treatments was completed at three dissolved oxygen (DO) concentrations: 8.1 (near air saturation), 5.6 and 4.5 mg/liter. For fish exposed to a mean toxicant concentration of 920 micrograms/liter at 8.1 mg/liter DO mean survival and mean weight was less than in controls. At the comparable toxicant concentration at 5.6 mg/liter DO, survival and growth of test fish were not statistically different (P = 0.95) from those of control fish. At the comparable mean toxicant concentration of 920 micrograms/liter at 4.5 mg/liter OD, more marked toxic effects on survival and growth occurred. No toxic effects on survival, growth, and bioconcentration were evident at 500 microrams/liter 1,2,4-TCB or lower concentrations.