The internal lethal concentration is a potential measure of toxicity which could be usefully applied in environmental toxicology and risk assessment. Using halobenzenes, which are common environmental contaminants, and represented test compounds, experiments were conducted in aquaria with the mosquitofish (Gambusia affinis). The average internal lethal concentration (ILC50) for four representative halohydrocarbons, 1,4-diBB, 1,2,3-triCB, 1,2,4-triBB, and pentaCB, were consistent with those previously observed, i.e., 2.3–8.3 mmol kg−1fish over exposure time periods of 10.4 to 633 hr. However, the ILC50for all the compounds is not constant but decreases with increasing exposure time period with a mean first-order rate constant of (4.21 ± 0.70) × 10−3hr−1. The time dependency of the ILC50is inconsistent with the critical internal concentration hypothesis which requires the ILC50to reach a constant critical value when lethality occurs. The life expectancy of the fish from the beginning of chemical exposure could possibly be related to the ILC50–exposure time relationship.