The host-parasite relationship between bogue (Boops boops) and Cerathotoa parallela (Isopoda: Cymothoidae) was studied in the Southern Tyrrhenian Sea (overall prevalence of isopods in the area, age-dependent and sex-dependent prevalence). Yearly prevalence ranged from 1.0% in March to 10.7% in October, with a positive correlation between prevalence and mean sea surface temperatures (SST), highest values being related to the period of increasing SST. The first four age-classes were parasitized, with prevalence decreasing according to the age of the host. The relationship between total length (TL) and total weight (TW) was examined in infested and non-infested specimens of the same age whereby slight differences were detected, i.e. at a given length non-infested specimens weighed more than infested fishes. The parasite occurred throughout the study area with wide fluctuations in prevalence, from 0 to 36.4%, the values being higher in the eastern part of the study area. C. parallela parasitize the bogue Boops boops in a manner that does not constitute a serious threat to the life of the host.