West African Diana monkeys (Cercopithecus diana) and Campbell's monkeys (Cercopithecus campbelli) frequently form mixed–species associations. Males of both species produce acoustically distinct alarm calls to crowned eagles (Stephanoaetus coronatus) and leopards (Panthera pardus), two of their main predators. Field playback experiments were conducted to investigate whether Diana monkeys respond to Campbell's alarm calls and whether they understand the calls' semantic content. Diana monkeys responded to playback of Campbell's leopard or eagle alarm calls as though the original predator were present. In a second experiment, Diana monkeys were primed with either Campbell's eagle or leopard alarm calls and then subsequently probed with the vocalizations of a crowned eagle or a leopard. Results showed that monkeys used the semantic information conveyed by the Campbell's alarm calls to predict the presence of a predator. The data are consistent with the hypothesis that non–human primates are able to use acoustic signals of diverse origin as labels for underlying mental representations.