Anguillicola crassus, a nematode parasitizing the swimbladder of the European eel (Anguilla anguilla) caused substantial mortality among eels in Lake Balaton in the years 1991 and 1992. Parallel to a 3-year study of the infection dynamics of eels, Anguillicola larval infection of paratenic host fishes was also surveyed in the lake between 1991 and 1993. During that study, a total of 1,382 specimens of 22 paratenic host fish species were processed. The results showed that anguillicolosis had become a parasitosis widespread throughout the lake, and larval infection could be detected in practically all paratenic hosts examined. The prevalence and intensity values recorded in the paratenic hosts do not completely follow the dynamic change observed during the survey of eel infection. During the study of larval infection in paratenic hosts, only inter-species differences in the prevalence and intensity of infection could be found, rather than the expected course of infection spreading from the West to the East in both space and time. Marked differences existed between paratenic host species in the degree of host reaction to the larvae.