Corrosion casts have shown that the systemic vasculature of various bony fishes displays 2 types of branching in all arteries, except those of the CNS and of the intestines: 1. the usual dichotome branching. 2. numerous narrow shunt vessels departing continuously from the primary arteries to feed a secondary arterial system which parallels the primary one. The secondary arteries receive their blood exclusively via the interarterial shunt vessels. They also branch in a dichotome manner towards their peripheral capillary bed. All secondary arteries are connected by 1 or 2 vessels accompanying the dorsal aorta. In the lumen of the primary arteries, the origins of the interarterial shunt vessels are bordered by microvilli-bearing a endothelial cells. These are expected to be involved in the regulation of blood flow to the secondary blood vessel system. The organization of the circulatory system of fishes is reviewed in the light of these facts.