Eels were sampled twice a month (March 1990 though March 1991) at the Kolenhaven (Albertcanal, Genk, Belgium) and examined for Anguillicola crassus nematodes in the swimbladder. Averaged over all samples, 90.2% of the 345 eels were infected with a mean intensity (averaged for all stages) of 17 nematodes. Thirty-one percent of all nematodes, which were adult worms or their remnants, were present in the swimbladder lumen, while the other 69% which consisted of the third or fourth larval stage and preadults were found in the swimbladder wall. No clear seasonal fluctuations in parasite prevalence or mean intensity were observed. Both L 3 -larvae and mature adult worms were present throughout the year. Eggs or L 2 -larvae were observed in 41% of the infected eels. This presence showed no seasonal pattern, though it was positively correlated with eel length: massive concentrations of L 2 -larvae were only found in eels bigger than 40 cm. A positive but weak correlation between eel length and number of adult nematodes in the swimbladder was significant, while no correlation was found between eel length and total parasite burden.