The structure of the opercular filament blood vessel of the serpulidPomatoceros lamarckii Quatrefages, has been described by electron microscopy. This blood vessel is in the form of a blindly ending, spirally twisted opercular blood vessel in the distal, basin-shaped operculum and in the form of a straight, tubular, peduncular blood vessel in the proximal stalk or peduncle. The fundamental structure of the blood vessel consists of an extracellular, luminal basal lamina and an outer layer of cells, well endowed with thick and thin myofilaments, referred to as myo-epithelial cells. The organisation of the myofilaments in the opercular blood vessel myo-epithelial cells resembles smooth muscle cells. By contrast, the organisation of the myofilaments in the peduncular blood vessel myo-epithelial cells resemble striated muscle cells. Scattered irregularly along the length of and closely associated with the luminal basal lamina are flattened, elongate coelomocyte-like cells. Similar cells are also found closely associated with the myo-epithelial cells in the connective tissue. Observations suggest that these cells may be implicated in the synthesis and/or endocytosis of chlorocruorin molecules.