The title selenocycle-fused tetrathiafulvalene derivatives (BES-TTF and DS-TTF) and tetraselenafulvalene derivative (BES-TSF) have been synthesized as novel electron donors, and their conducting molecular complexes have been studied. Although DS-TTF formed only semiconducting complexes, BES-TTF and BES-TSF gave metallic complexes with various electron acceptors, such as TCNQ, ClO4-, PF6-, and AsF6-. Among them, the TCNQ complex of BES-TSF showed an extraordinarily high room-temperature conductivity of 2700 ± 500 S cm-1, which is of the highest class for a molecular complex. The complexes of BES-TTF underwent a typical metal-to-insulator transition at low temperature, characteristic of one-dimensional organic metals. On the other hand, complexes of BES-TSF were less temperature-dependent and remained highly conducting, even down to cryogenic temperature. The different behaviors of the three donors are discussed on the basis of the crystal structures of their representative complexes as elucidated by X-ray crystallographic analyses.