Some land-based marine fish-farms situated on the Atlantic coastline of France use high volumes of underground sea water. Studies of the available quantities and movements of this underground resource became necessary, using fluorescent dyes such as fluorescein. As fluorescein may reach reared fish, it became important to assess its toxicity to fish. Acute fluorescein toxicity to turbot (Scophthalmus maximus) was investigated by exposing fish to different fluorescein concentrations (0.5, 0.7, 0.9, 1.1 and 1.3 milligrams) at 14.0 C for 24, 48 or 96 h. The lethal concentration inducing 50% fish mortality (LC50) was 997.1 +/- 11.4 mg/l (mean +/- standard deviation) after a 24, 48 or 96 h exposure. Toxicity affected the central nervous system. Early postmortem findings were a brown-green coloring of some tissues and encephalon congestion. The fluorescein LC50 was much higher than the recommended concentration in field applications (1 mg/l), indicating that fluorescein toxicity to turbot will not be expected when used at the recommended concentration.