A sample of fluid such as a gaseous fluid leaving a liquid/gas separator in a surface installation of an oil well, is taken continuously via a sampling tube. The tube opens out on the axis of a gas outlet pipe, facing towards the separator receptacle and located at the junction between the pipe and the receptacle. Sampling is performed isokinetically by regulating the flow rate in the tube by means of a regulator valve controlled by a regulator as a function of flow rates measured in the tube and in the pipe. In the tube, liquid is separated from the gas by means of a filter.