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In the triclinic crystals ammonium and potassium tetroxalate dihydrate 34 and 31 sound velocities respectively were measured by means of the diffraction of light by ultrasonic waves. From these velocities the 21 independent components of the elastic tensor were calculated. Thus, the method first tested by Haussiihl & Siegert on CuSO4.5H20 was successfully applied to two other triclinic crystals. The compounds under investigation were found to exhibit an extremely high elastic anisotropy.