The occurrence of larvalAnisakisin North Sea herring was examined during the period 1965‐1972 using a sampling method which yielded comparable data for all eight years. Data on abundance of infestation in several herring stocks pointed to remarkable fluctuations over the study period with an increase in the period 1966‐1968 followed by a decrease in the period 1968‐1972. Of various hypotheses that may explain this situation, it is concluded that changes in the migration behaviour of the herring may be responsible. Other data and statistics are discussed with special reference to variations in infestation level in different samples and to the occurrence of adult nematodes in marine ma