A continuous-flow system was developed for evaluating the effects of insoluble and volatile organics on the developmental stages of fish. A closed test system, devoid of standing air space, was used to minimize volatility as a variable. Insoluble compounds were suspended in influent water by mechanical homogenization, without the use of carrier solvents. Tests were performed with aniline, chlorobenzene, and phenol. Water hardness was maintained at 50 and 200 mg/litre calcium carbonate, and exposure was continuous from fertilization through four to eight days after hatching for largemouth bass, channel catfish, goldfish, and rainbow trout. The results indicated that there was good reproducibility of exposure concentrations down to 1 µg/litte or less. For a calculated concentration of 1 µg/litre phenol, the actual test-water concentrations (mean ± standard error) for four replicates were 0.7 ± 0.2, 1.2 ± 0.3, 1.3 ± 0.3, and 1.5 ± 0.3. When phenol was administered in hard water, the LC₁ and LC₅₀ values determined at hatching were 0.2 and 80 µg/litre for trout, and 2.0 and 710 µg/litre for goldfish. Phenol was less toxic in soft water. Chlorobenzene at 0.09 mg/litre produced complete mortality of trout eggs. When bass eggs were exposed through four days after hatching, the LC_50s for chlorobenzene ranged from 50 to 60 µg/litre. The aniline LC₅₀ values calculated for catfish, goldfish, and bass eggs treated in soft water were 5.6, 10.2, and 47.3 mg/ litre, respectively. Because of the differential rates of larval mortality for the three species, the LC₅₀ range narrowed considerably when the exposure time was increased beyond hatching. Water hardness exerted no appreciable effects on the toxicity of either chlorobenzene or aniline. All three organic compounds produced significant frequencies of teratic larvae.