Ceratothoa oestroides is reported causing mortality outbreaks in cage cultured sea bass (Dicentrarchus labrax) in Croatia. This parasitism and its effects can cause high losses in juvenile populations of cultured D. labrax and sea bream (Sparus aurata [Pagrus aurata]). In older and market sized fish, bass in particular, the effect of parasitism is mainly related to growth retardation. The source of the parasite and route of infection seems to be by contact with wild fish in the cage environment.