TbMn 6 Ge 6 - x Ga x compounds (x=0.1, 0.2, 1.0) were studied by magnetization measurements and neutron diffraction. The three compounds order ferrimagnetically at 449, 449 and 406 K, respectively, and display various transitions at lower temperatures. TbMn 6 Ge 5 . 9 Ga 0 . 1 displays a conical structure below 71 K (μ T b =8.71(9) μ B , μ M n =2.24(5) μ B at 2 K) and a helimagnetic structure from 71 K to room temperature, where an additional ferrimagnetic component occurs. TbMn 6 Ge 5 . 8 Ga 0 . 2 is ferrimagnetic below 102 K and above 212 K, and displays a complex magnetic structure where commensurate and incommensurate components coexist in the intermediate temperature range. At 2 K, the moments are aligned along the c-axis (μ T b =8.5(1) μ B , μ M n =2.24(5) μ B at 2 K) and are perpendicular to the c-axis at 300 K. TbMn 6 Ge 5 Ga is ferrimagnetic over the whole ordered range and displays a moment reorientation process at 214 K. At 2 K, the moments (μ T b =9.0(1) μ B , μ M n =2.24(6) μ B ) are aligned along the c-axis and they are close to the (001) plane at 300 K. The results are discussed and compared with the behavior of the other RMn 6 Ge 6 - x Ga x compounds (R=Ho, Tm, Lu).