this paper describes the interaction between two species: white anglerfish lophius piscatorius and its specific leech parasite calliobdella lophii off the north of spain. catches of white anglerfish by the spanish gill-net fleet operating in those waters were sampled monthly in 2001. abundance, mean intensity and prevalence were used to analyse infestation level by c. lophii. prevalence of leeches and fish length was significantly correlated. mean intensity was higher on larger white anglerfish. significant differences were found in the presence of leeches by fish length between areas, with higher values in the western part of the studied area. no significant differences were found in the presence of leeches between sexes or seasons.