29 breeding male three-spined sticklebacks (Gasterosteus aculeatus) caught in the wild at different times in the breeding season were exposed simultaneously to a potential threat to their nest (a conspecific male) and to two hunting trout (potential predators of adult sticklebacks, seen through a transparent partition). By promoting a variety of protective responses, the presence of the predators reduced both the time spent confronting the intruder and the rate at which it was attacked. Subjects with a clutch of eggs in their nest maintained higher levels of territorial defence in the presence of predators than did those with an empty nest. However, those breeding later in the season took fewer risks to defend their nest. Possible proximate mechanisms responsible for these results are discussed. In addition, the adaptive significance of the behavioural changes is considered in the light of the value of the brood and the expected future reproductive output of the sticklebacks breeding at different times in the season.