A probe made of a hollow glass cylinder is placed with its axis parallel to a slightly ionized, high speed flow. Biased electrodes on the inside surface collect the ions and the electrons entering the probe. The charged particle density is obtained by dividing the collection current by the product of the probe entrance area, the gas velocity, and the charge per particle. In order to establish that all charged particles entering the probe are collected, it is verified that no particles are attracted by a pair of auxiliary electrodes downstream of the main ones. Proper functioning also requires negligible charged particle production inside the probe, and furthermore that the velocity of the charged particles at the probe entrance be equal to the flow velocity. The probe was used to obtain ion density profiles behind shock waves of Mach numbers 6 to 9 in air at initial pressures of 50 to 400 μ Hg. Ion densities measured were in the range 107 to 1010 cm−3. Above this range, total collection was prevented by space charge effects. The equilibrium ion densities measured are in satisfactory agreement with theory. The values obtained for the ionization relaxation time are probably influenced by the presence of impurities.