The tricontinuous structure of ABC-type triblock copolymer in bulk at equilibrium was studied by both transmission electron microscopy (TEM) and small-angle x-ray scattering (SAXS). Sample is an anionically polymerized isoprene-styrene-2-vinylpyridine triblock copolymer whose volume fraction is 0.22-0.59-0.19 and total molecular weight is 64 000. It was found that computer-simulated images based on tricontinuous structure with two parallel surfaces to the gyroid surface, which is one of the three-dimensionally infinite minimal surfaces agree well with the observed TEM images. Further, calculated diffraction patterns for the same tricontinuous structure well represent the observed SAXS intensity maps on imaging plates. These facts imply that the structure of the present triblock copolymer is the tricontinuous double gyroid structure, whose space group is I4(1)32. (C) 2000 American Institute of Physics. [S0021-9606(00)51210-0].