Investigation of 2197 cod larvae and post-larvae collected in the North Sea revealed high prevalence of infection with a parasitic anisakid nematode identified morphologically and genetically as Hysterothylacium aduncum. Nematodes were third stage larvae and were almost exclusively found in the body cavity and they were never encapsulated. Prevalence increased significantly from 1992 to 2001 concomitantly with increased sea temperature. The possibility that the extent of parasitism is influenced by temperature change is discussed.