A corrosion cast of the entire blood vascular system was made by a single injection of resin from the heart of carp and a Japanese catfish. Either semipolymerized methyl methacrylate or Mercox CL with 30-50% methyl methacrylate monomer was used as the resin preparation. The latter was better for photographing. Both were polymerized by addition of 1% benzoyl peroxide.The segmental blood vessels of carp and a catfish showed an alternating arrangement of artery and vein in successive segments with some irregularity.