Thus far, high burdens of Triaenophorus crassus plerocercoids have been reported only in old age groups of coregonid and salmonid fishes. Here we show heavy infection with T. crassus in young whitefish Coregonus lavaretus in the ultra-oligotrophic and regulated Achensee in Tyrol, Austria. Prevalence of T. crassus on C. lavaretus was 100% in all age groups and abundance significantly increased with fish age. The mean annual accumulation of T. crassus was 5.2 parasites in 0- to 7-year-old C. lavaretus, and 2-year-old specimens already harboured a mean of 19.4 plerocercoids. In Arctic charr Salvelinus umbla, however, the prevalence of T. crassus was less than 16% and the majority of infected fish contained only one or two plerocercoids. Triaenophorus nodulosus was present neither in C. lavaretus nor in S. umbla. We assume that the heavy T. crassus infection in C. lavaretus is largely related to their zooplankton-dominated diet and to the characteristics of Achensee, while habitat choice and feeding strategy of the S. umbla population are seen to be the main reasons for their low burdens of T. crassus.