Infection of Gymnophallus sp. in the bearded horse mussel (Modiolus barbatus) was studied in samples collected during 1 year in the Mali Ston Bay (eastern Adriatic Sea). Mean prevalence of sporocyst was low (3.5%), which peaked in February (7.1%), whereas the mean metacercariae prevalence was 8.1%, peaking in September (16.7%). Histologically the Gymnophallus sp. sporocyst infection in bearded horse mussel resulted in clear reaction of bivalve tissue, where the induced changes depend strongly on the site of trematode infection. Retardation of gametogenesis, necrosis of connective tissue and hemocytic infiltration are the main histological features of infection in this new host.