[en] We studied the low-temperature thermoelectric properties of single crystals of Mg_3Sb_2_−_xBi_x (0 ≤ x ≤ 2) grown by the Bridgman method. The crystals are well aligned along the hexagonal c axis as documented in the huge anisotropy of the electrical resistivity; ρ_/_/_c/ρ_⊥_c = 100 for x = 2. Upon increasing x, the semiconducting behaviour in ρ(T) changes to the metallic behaviour, and the Seebeck coefficient S(T) at room temperature decreases from 590 μV K"−"1 for x = 0 to 67 μV K"−"1 for x = 2. Although the thermal conductivity at 300 K is as low as 0.88 W mK"−"1 for x = 0.7, the large resistivity of 1 Ωcm leaves the figure of merit, ZT, at a low level of 5.8 × 10"−"3 at 300 K. (paper)