The growth rate reduction of mosquito fish (Gambusia affinis) fry was investigated with a range of sublethal exposure levels of four halobenzenes for 42 days. These compounds were found to produce growth rate reduction in mosquito fish fry at concentrations as low as 0.30, 0.18, 0.025, and 0.010 μmol liter−1for 1,4-dibromobenzene, 1,2,3-trichlorobenzene, 1,2,4-tribromobenzene, and pentachlorobenzene, respectively. The aqueous exposure concentrations causing growth rate reduction of 50 and 10% (EC50and EC10, respectively) for the halobenzenes were 0.067–3.4 and 0.0042–0.32 μmol liter−1, respectively. The EC50and EC10values are within the ranges of 5 to 8% and 0.1 to 3.9% of the LC50values, respectively. The percentage of growth rate reduction relative to the LC50could possibly be used to describe chronic toxicity effects of organic compounds with the aquatic organisms. The internal concentrations obtained from the analysis for the halobenzenes were generally consistent with the calculated internal concentrations. The lipid-based internal concentrations that gave 50 and 10% growth rate reductions were 8.3–27 and 0.5–1.6 mmol kg−1, respectively. These values have a more limited range than the corresponding external aqueous concentrations. The quantitative structure–activity relationships between the internal concentrations at 50 and 10% growth rate reduction and physicochemical parameters were found to be less satisfactory than those based on external aqueous concentrations.