This paper presents a method for determining the electron density and electron collision frequency in a cylindrical plasma column by placing an rf coil coaxially around the plasma and measuring the coil impedance. Simplified expressions relating the impedance to plasma parameters are found to be valid under certain suitable conditions. The plasma of a hot‐cathode low‐pressure dc discharge in argon has been studied by an rf coil probe. The dependence of electron collision frequency on electron velocity has been taken into account. The values of the electron density and average collision frequency, as obtained from the rf probe under different discharge conditions, are in fairly good agreement with those given by a cylindrical Langmuir probe.