The ultrastructure of endothelial cells and rodlet cells in the bulbus arteriosus of specimens representing six genera of Cichlidae is described. The former are very closely packed by membrane–bound and mainly electron–dense inclusion bodies (0.3–0.7μm).InApistogramma ramireziI observed numerous subendothelial rodlet cells throughout the entire length of the bulbus arteriosus. These cells penetrate the endothelium and connect to the latter by desmosomes and tight junctions. The luminal part of the cell contains numerous vesicles and tubules (width 50–100 nm), whereas the basal part is occupied by a number of membrane–bound, club–like inclusions (length ≤ 5 μm). Between these two layers there occurs a layer of small, elongated mitochondria. Peripherally, these cells consist of a filamentous wall, except in the apical area.The endothelial and rodlet cell inclusion bodies do not react with phosphotungstic acid (pH 1) or Sudan black B stain. The endothelial cells react strongly with periodic acid–Schiff (PAS) stain, whereas the rodlet cells are only moderately coloured by this stain.The present results are discussed and compared with those reported previously for endothelial/ endocardial cells and rodlet c