In response to disturbances in their surroundings, Barbary macaques, Macaca sylvanus, utter a distinctive call, the 'shrill bark'. Differences within this call type correlate with the stimulus eliciting the calling. I tested whether monkeys categorized calls according to the eliciting stimulus and whether their perception of calls coincided with the classification derived from the acoustic analysis. Different playback designs using the habituation-dishabituation paradigm were created in which calls exhibiting varying degrees of acoustic difference were paired. I conducted experiments in two populations of semi-free ranging Barbary macaques. The results suggest that subjects categorized calls according to the eliciting stimulus. In addition, subjects from the population in which the recordings were made were significantly better than subjects from the other population at discriminating between calls with small acoustic differences. The results suggest that call categorization is influenced by experience, mediated by individual knowledge of the caller or common call characteristics within the population. Copyright 1998 The Association for the Study of Animal Behaviour. Copyright 1998 The Association for the Study of Animal Behaviour.