An investigation of the oil spill after the wreck of the tanker ‘Erika’ on 16 December 1999 showed that the maritime area around the town of Le Croisic (between the Vilaine and Loire river estuaries) was severely contaminated by the resulting black tide. Samples of sediment, oil and water were collected during several missions to the Le Croisic headland and analysed. The toxicity of the oil was tested on two organisms, the harpacticoid copepod Tigriopus brevicornis and the alga Enteromorpha intestinalis. The tests showed only slight degradation of the oil after three months. In laboratory conditions, the toxicity of the sediment contaminated by the oil was apparent on the copepod after four days of exposure. Seawater in contact with the oil also had an effect on the alga.