Host–parasite associations are among the primary drivers of evolutionary diversification, and hold considerable importance for understanding ecological equilibria. In particular, crustacean ectoparasites are typically associated with many fish families, and may, under certain conditions, pose threats to fisheries and aquaculture. Cymothoid isopods include blood-feeding genera that inhabit the mouth of their host, and whose variation in life-history strategies remains largely unexplored. Here we investigate the size relationship between the highly prevalent Ceratothoa italica and its main natural host, the striped sea bream, Lithognathus mormyrus. We found significant correlation between host size and that of both female and male parasites. Although the generality of a host–female size association in mouth-dwelling cymothoids had been widely recognised for some time, here we provide the first robust support for the occurrence of this size association also in mouth-dwelling male parasites. The potential underlying biological causes of the patterns are discussed, contributing to the debate on the evolution of host–parasite interactions, and the adaptive radiation of this family of parasitic isopods.