The effect of Anguillicola crassus (Nematoda) on gas composition in the swimbladder of eels (Anguilla anguilla) was studied using mass spectrometry in feral eels from two German rivers and in experimentally infected eels. In both naturally and experimentally infected eels significant correlations were observed between the proportion of oxygen in the swimbladder and level of infection with Anguillicola crassus. In swimbladders of naturally infected feral eels the contribution of oxygen to swimbladder gas was reduced by 36-62.9% and in experimentally infected eels it was reduced by 11.4-57% compared to uninfected controls. The proportion of CO2 appeared to be lower in infected swimbladders compared to uninfected ones. However, this change was not significant. The findings are discussed in relation to an altered structure of the swimbladder wall due to the parasite.