Selenium-substituted TTM-TTP (2,5-bis[4,5-bis(methylthio)-1,3-dithiol-2- ylidene]-1,3,4,6-tetrathiapentalene) derivatives (1a-1d), in which several 1,3-dithiole rings of the bis-fused TTF framework are replaced by 1,3-diselenole rings, have been prepared. In particular, we have accomplished the first selenium substitution of the inner TTP part (1a). The 1:1 composition iodine salt, (1a)I3, is an insulator constructed of the donor trimers. The 1:1 GaCl4 salt, (1a)GaCl4, has a uniform column isostructural to the sulfur analog, (TTM-TTP)FeBr1.8Cl2.2; it exhibits metallic conduction down to about 60 K, the lowest metal-insulator (M-I) transition temperature in 1:1 salts. The donors 1c and 1d give the iodine salts isostructural to the sulfur analog, (TTM-TTP)(I3) 5/3. By the selenium substitution, the M-I transition of (1d)(I 3)5/3 is entirely suppressed.