Experimental results are presented from negatively‐biased cylindrical Langmuir probes located in a magnetohydrodynamic (MHD) generator with the flow supersonic and the plasma both in equilibrium and uniform. Voltage‐current characteristics were obtained over a wide range of bias voltages, up to the arcing limit, and these showed that the probes operate in the convection‐sheath mode. Electron densities deduced from the probe measurements are in agreement with values obtained by other methods; the results confirm the applicability of the Langmuir probe as a diagnostic technique in flowing plasmas.