Eavesdropping on the alarm calls of other species can provide vertebrates with valuable information about danger, and often responses to heterospecific calls seem to be acquired through learning. However, animals might not require learning to respond to heterospecific alarm calls that are acoustically similar to conspecific calls. Although previous work suggests that learning is necessary for superb fairy-wrens to develop responses to heterospecific alarms, acoustic similarity might also be important. We tested whether fairy-wrens responded to playback of unfamiliar alarm calls and if the strength of responses was affected by acoustic similarity to conspecific calls. We then determined which acoustic properties were likely to be used in alarm call identification. Birds fled to cover after playback of calls that were acoustically similar to their own but did not usually respond to less similar calls. Fairy-wren aerial alarm calls are high-frequency and rapidly frequency modulated; their probability of fleeing to playback of unfamiliar calls increased in response to calls with increasing peak frequencies, and they spent more time in cover following calls with a number of frequency cycles similar to their own. Fairy-wrens also responded strongly to relatively dissimilar calls of 1 allopatric species, possibly because these unfamiliar calls resembled those of a sympatric species to which they had learnt to respond. Our study shows that acoustic similarity can prompt responses to heterospecific alarm calls regardless of experience and together with previous work suggests that both acoustic similarity and learning are important for interspecific responses to alarm calls.