The effect of the parasitic swimbladder nematode, Anguillicola crassus, on the physiological status of wild European eels (Anguilla anguilla) was investigated during an 18 month survey (February 1995 until September 1996), and compared with that of wild uninfected eels collected over the same time-period. Despite the occurrence of up to 15 blood-feeding adults in the swimbladder lumen of the infected eels and as many as 25 additional larvae in the swimbladder wall, there were no major differences in hormonal, metabolic or osmoregulatory status of the 2 groups of eels. Wild European eels appear to adapt to chronic parasitism with Anguillicola crassus. The possible adverse effects of additional simultaneous stressors are discussed.