To determine whether toxic metabolites produced by fungi could cause shellfish toxicities, mussels were contaminated in laboratory conditions by sterile filtrates of a liquid culture of a strain of the fungus Trichoderma koningii previously isolated from a shellfish, the cockle (Cerastoderma edule). Mussels were kept in aerated natural seawater and fed with a culture of the microalga Isochrysis galbana, to which a filtrate of liquid fungal culture was added. Mussels were exposed to contamination for 7 days at 16 or 20 degrees C and extractions were then performed and their activity tested on blowfly larvae. The same toxicity was found in the fungal filtrate and the shellfish, indicating bioaccumulation. The digestive gland was the most toxic part of the mussel, confirming contamination by filtration. Treated mussels produced a mucus which appeared to be a means of eliminating toxic metabolites.