We propose a novel method to measure in situ the bulk speed of a space plasma. It is based on the analysis of the electrostatic field spectrum produced by the Doppler-shifted thermal fluctuations of the plasma ions which can be measured with a sensitive receiver at the terminals of a passive electric antenna. We present a preliminary application in the solar wind using the data acquired in the ecliptic plane by the Unified Radio and Plasma experiment (URAP) on the Ulysses spacecraft. This should allow us to extend to the bulk speed the method of thermal noise spectroscopy which already gives an accurate in situ diagnosis of the electron density and bulk temperature. This method can be complementary to classical electrostatic analyzers for both interplanetary and magnetospheric studies. ¿ American Geophysical Union 1996