In the summer of 1995, 30 tonnes of eel (Anguilla anguilla) died in Lake Balaton, Hungary. An investigation was carried out to find the causes of this ecocatastrophe. During this investigation, certain biochemical parameters, i.e. the blood sugar level, the acetylcholinesterase (AChE, EC 3.1.1.7), lactate dehydrogenase (LDH, EC 1.1.2.3), glutamic-oxaloacetic transaminase (GOT, EC 2.6.1.1), and glutamic-pyruvic transaminase (GPT, EC 2.6.1.2) activities in the blood serum of the collected surviving and dying eels were examined. Deltamethrin, the active ingredient of the insecticide K-OTHRIN 1 ULV, used against mosquitoes was detected in different animal species, i.e. eel, bream (Abramis brama), pike perch (Stizostedion lucioperca), and the common gull (Larus canus) and in sediment samples from the lake. Additionally, laboratory experiments were carried out to study the effects of deltamethrin on eels. During the investigation in the field it appeared that the AChE activity was significantly lower in the blood serum of the dying eels as compared to that in living animals (P<0.05, Student's t-test). The blood glucose content exhibited a difference, too: it was 2.5 times higher in the dying eels than in the surviving ones. A huge increase in the LDH level was measured in the dying eels. The GOT activities of the serum were twice as high in the dying eels as in the living fish, while the GPT was not significantly changed. Deltamethrin was detected in different tissue samples of the dying eels: 2.70-18.1 microg/kg in the liver, 9.0-31.1 microg/kg in the gill and 3.0 microg/kg wet tissue in the muscle. Deltamethrin residues were found in tissue samples from other animals, in the following concentrations: 0.44 microg/kg in bream, 2.14 microg/kg in pike perch and 1.06 microg/kg wet tissue in dead gulls. The sediment samples collected from the sites of the devastation contained deltamethrin in a concentration of 5.50-30.00 microg/kg wet sediment at the time of the eel deaths, and in a concentration 7.00-8.75 microg/kg wet sediment a month later. Laboratory experiments with the insecticide K-OTHRIN 1 ULV revealed that 1.00 microg/l of its active ingredient, deltamethrin, caused the death of 50% of the eels after an exposure time of 96 h. During this experiments similar trends could be observed in changes of enzyme activities of the treated eels to those that were detected in filed study during the eel devastation in Lake Balaton. At the end of a one-week treatment with the insecticide at the concentration of 0.5 microg/l of its active ingredient the gills of the treated eels contained deltamethrin at 12.6-44.8 microg/kg wet tissue concentration, while at the 24th hour after the treatment (11.2-42.7 microg/kg wet tissue) deltamethrin concentration in the liver of treated eels could be detected. All the above-mentioned changes and the detected deltamethrin residue in the eels appear to demonstrate the contribution of deltamethrin to the severe eel devastation. This information on the ecological risk of pyrethroid insecticides might be useful in their further application.