The effects of five nematocidal drugs (levamisole HCl, metrifonate, fenbendazole, mebendazole, and ivermectin) on the nematode Anguillicola crassus, pathogenic in eels and recently introduced in Europe, were tested under in vivo conditions. The resulting tissue alterations were studied by means of light and electron microscopy. It was found that levamisole and metrifonate were most effective in a freshwater bath (1 mg/l) for 24 h. The 50% lethal dose of levamisole was 250 mg/l per 24 hours, whereas that of metrifonate was only 10 mg/l per 2 hours. Morphological studies of the nematodes showed that there was no drug-specific reaction. In general, the hypodermis, the cytoplasmic portions of the muscle cells, and the intestinal wall were most intensively damaged, leading to an irreversible vacuolation and finally to death.