Jamming experiments were conducted to assess the effectiveness of several different types of signals for interfering with the ability of echolocating bats (Eptesicus fuscus) to discriminate target range. Random interfering noise covering the bat's sonar bandwidth of 25–50 kHz must be sufficiently intense for the echo signal‐to‐noise ratios to approach 0 dB before marked breakdown of echolocation occurs. This is true for continuous noise, intermittent noise, and for noise incorporating internal correlation at the same period as the travel time of sounds going to and from the targets. FM interference is 30 to 40 dB more effective in jamming the bat than random noise of the same spectrum. The results suggest that the bat's sonar receiver applies frequency‐sweep criteria to incoming sounds to decide whether potential echoes are present. Neurons with properties appropriate to this filtering function, analogous to motion detection in vision, are known to exist in the bat's auditory nervous system.