An ethylenedioxy derivative of a bis-fused donor composed of tetrathiafulvalene and tetraselenafulvalene (EO-ST-STP) was successfully synthesized. X-ray structure analysis revealed that (EO-ST-STP)(2)X (X=PF6 and ClO4) adopted the so-called beta-type molecular packing. A tight-binding band calculation suggested these salts had a closed Fermi surface. Both salts exhibited high conductivity of sigma(rt)=990-1200 S cm(-1) and metallic temperature dependence down to T-MI=50-70 K.