Behavioural plasticity is expected to be favoured in risky environments, such as when prey species coexist with predators because prey must alternate between fitness-related foraging/mating behaviours and antipredator behaviours that enhance survival. We compared behavioural plasticity in Trinidadian killifish that are found in sites with and without predators. We quantified aggressive and antipredator behaviours via a mirror assay in second-generation lab-reared and wild-caught killifish before and after exposure to predator cues. We compared 2 types of aggression including: overt aggression (ramming, biting, lunging, and tail-slapping) and display aggression (spine arching, bending into an s-shape, and opercular flaring). We additionally compared the amount of time the fish spent frozen as a proxy for antipredator behaviour. We show clear differences in plasticity between populations with and without predators. Killifish from sites with predators decreased overt aggression in response to exposure to predator chemical cues. Plastic responses to the predator cue were lower in killifish from sites that lack predators. Interestingly, wild fish from sites without predators did respond to the predator cue by decreasing overt aggression and increasing time spent frozen, though to a lesser degree compared to the fish from sites with predators. Our results support the expectation that development in a risky environment favours evolutionary changes in predator-mediated behavioural plasticity.