The ability of the nematode Anguillicola crassus to infect eel larvae (glass-eel stage) was tested. The results show that glass-eels fed on infected copepods, the natural intermediate host of the nematode, can be infected. Light microscopical examination of the infected developing swimbladder tissue revealed that the infection results in a significant thickening of the connective tissue. The basolateral labyrinth of gas gland cells is very much reduced in infected swimbladders, and the distance of gas gland cells to blood capillaries is enlarged. Critical swimming speed, defined as the speed where the larvae were no longer able to swim against the current, was similar in infected and uninfected animals. At intermediate speeds (about 60-80% of critical swimming speed) infected eels showed a slightly higher swimming activity than control animals. Resting oxygen consumption, measured as an index of metabolic activity, within the first 2 months of infection was higher in control animals, which may be due to a reduced rate of activity in infected glass-eels. By 4-5 months after the infection, however, it was significantly higher in infected animals. This may indicate that at this stage a higher activity of the animals is required to compensate for the increase in body density, but swimming performance of infected and non-infected glass-eels was not significantly different. Oxygen consumption during swimming activity, measured in a swim tunnel at 50% of maximal swimming speed, also was not affected. The results thus show that even glass-eels can be infected with A. crassus, and this probably contributes to the rapid spread of the nematode in Europe. While aerobic metabolism during swimming activity is not affected at this stage of infection, the swimbladder tissue shows severe histological changes, which most likely will impair swimbladder function.