Long-range synchronization of neural activity is thought to contribute to fear behaviour. Karalis et al. show in mice that sustained synchronized 4 Hz oscillations in the dorsal medial prefrontal cortex (dmPFC) and the basolateral amygdala (BLA) predict internally generated freezing behaviour (in the absence of a cue) and cue-induced freezing behaviour following fear conditioning training, but they do not predict periods of passive immobility that occur prior to fear conditioning. Individual neurons in the dmPFC and BLA synchronized their firing to 4 Hz oscillations during freezing behaviour following fear conditioning, and optogenetically driving dmPFC neuronal activity at 4 Hz increased the number of freezing bouts. 4 Hz oscillations in the prefrontal–amygdala circuit may therefore be involved in the initiation and maintenance of fear behaviour.