The ultrastructure of the ventral aorta of Poecilia reticulata is described in larvae of variable age. In postnatal larvae this structure is located within the ventral pharyngeal wall. The tunica media consists of elongated smooth muscle cells, surrounded by a network of elastic fibrils (10 to 20 nm). The muscle cells are rich in free and membrane associated ribosomes, and contain a well developed Golgi apparatus surrounded by membrane-bound, electron dense bodies. During the first 21 d of postnatal life, the number of ribosomes declines gradually, whereas there is a corresponding increase in the amount of contractile material. In addition, the number of intercellular, elastic fibrils seems to increase with advancing age. No elastic lamina was seen in the present tissue, but in the 21 d old larvae the subendothelial elastic fibrils are sometimes aligned and may therefore make up an elastic lamina at a later stage of development. Probably, the elastic fibrils are composed of a central string (2-4 nm) of unknown chemical nature, covered by a layer of elastin. Thus, the elastic fibrillar network in young and adult bony-fish may be regarded as heavily reticulose elastin laminae, resembling those of the early fetal aorta in mammals. The present intercellular elastin is shown to be intensely stained with phosphotungstic acid at low pH and may therefore be rich in basic proteins. In 3 mm prenatal larvae the heart and the posterior part of the aorta are located at the outside of the body, whereas the anterior part occurs in the ventral pharyngeal wall. The aortic endothelium and tunica media consist of undifferentiated cells.(ABSTRACT TRUNCATED AT 250 WORDS)