During fear conditioning, animals learn to associate a previously neutral sensory stimulus, such as a particular odour (termed the conditioned stimulus), with an aversive experience (typically a footshock). Such fear learning alters processing in higher brain regions, but little is known about its impact on the sensory neurons that provide the initial input to sensory pathways. Here, the authors show that fear conditioning enhances the synaptic output of olfactory sensory neurons that is evoked by the conditioned stimulus. Thus, plasticity occurs at the first stage of sensory processing and may act to increase an animal's sensitivity to threat-predictive stimuli.