Infections with metacercariae ofBucephaloides gracilescensin the four‐bearded rocklingEnchelyopus cimbriuswere studied for a year in a Norwegian fjord. The prevalence was at all times high, but abundance showed a marked decline in spring to a low level during summer. A rise in abundance, variance‐to‐mean ratio and frequency of smaller metacercariae in early autumn (August‐September) shows infection to have occurred mainly in autumn. There were no differences in level of infection between host sexes, but a highly significant positive correlation of parasite burden with host length. No influence on the condition factor of the host was revealed, the parasite cysts in the central nervous system are believed to have low pathogenicity. A detailed account of the distribution of metacercarial cysts is given. No support for infection being via the lateral line canals is revealed, cercarial penetration is believed to be through the general surface of the head and through the unpaired fins of the trunk a