Sporocysts of the Fellodistomidae trematode Bacciger bacciger were embedded in gonadal tissue of the bivalve Donax trunculus from the Mediterranean coast of Spain. The overall prevalence of infection was 8.4%, increasing with host size to 23% in D. trunculus from 35 to 36 mm in shell length. Individuals <19 mm long were not infected. No clear seasonality was recognized in prevalence of infection. The proportion of infected individuals of both sexes was similar (6.56% in males and 8.04% in females), although the highest prevalence was in clams of unknown sex (13.47%). The proportion of infected clams reduced to virtual castrates of unknown sex reached 28%. The tegument of the sporocyst and shed cercariae was studied by light and electron microscopy. Scanning electron microscopy revealed a smooth surface on the sporocyst tegument, the area of which was greater due to the presence of circumferential infolds and numerous pits. A birth pore on one side of the sporocyst, uniciliate sensory organs and cup-shaped sensory-like structures were present on the tegument. The cercarial body had spines and uniciliate sensory organs. The ventral sucker of the cercariae had 2 rings of uniciliate sensory organs and tegumental spines

10 pages, 15 figures, 2 tables