Abstract: The apparatus for determining the pressure of capillary water in the ground, especially in soil, is of a two-part design and comprises a probe tube and a pressure measuring device. The pressure is measured by means of an elastically deformable membrane. For pneumatically coupling the pressure measuring device with the probe tube the latter is closed by a pierceable and self-sealing closure member. A hollow needle suitable for piercing the closure member is attached to the pressure measuring device. Either at or in the elastically deformable membrane there are arranged mechanical-electrical transducers which can be connected to an electric measuring and evaluation instrument for indicating the pressure which corresponds to the deformation of the membrane.