The various noise contributions of an rf SQUID have been studied. The importance of the noise figure of the rf amplifier has been pointed out by calculation and experiments, particularly by using a cooled GaAs FET preamplifier (noise temperature 25 K at 300 MHz) we have designed. The effects of noise upon the flux‐locked loop characteristic have been investigated by introducing the notions of unlocking probability and of unlocking average time; the improvement of performances, especially of the slew rate of the flux‐locked system by an increase of the modulation frequency, is limited by the noise.