(\mathrm{Mn}{1-s} T{x} \mathrm{Ge}{4}) (T: (\mathrm{Co}, \mathrm{Fe}) ) solid solution was synthesized at (6 \mathrm{GPa}) and (650-750^{\circ} \mathrm{C}) using a belt-type highpressure apparatus. The crystal structure of the (\mathrm{Mn}{1-x} T_{5} \mathrm{Ge}{4}) solid solution is related to the (\beta) - (\mathrm{NiHg}{4}) type. The following structural changes were observed in the (\mathrm{Mn}{1-x} \mathrm{Co}{x} \mathrm{Ge}{4}) system: tetragonal ((x=0) ) to orthorhombic ((0<x<0.2)) and orthorhombic to pseudotetragonal ((0.2 \leqq x \leqq 0.4)). Similar structural changes were also observed at (0<x<0.4) (tetra. (\rightarrow) ortho.) and (x=0.4) (ortho. (\rightarrow) pseudotetra.) in the (\mathrm{Mn}{1-x} \mathrm{Fe}{x} \mathrm{Ge}{4}) system. These results indicate that the structural changes in (\mathrm{Mn}{1-x} T{x} \mathrm{Ge}{4}) strongly depend on the average valence electron number of the transition metal atoms. (\mathrm{Mn}{1-x} T_{x} \mathrm{Ge}_{4}) solid solutions are itinerant electron ferromagnets and the Curie temperature decreases linearly with decreasing unit-cell volume. 1993 Academic Press. Inc.