Although antipredator behavior of threespine stickleback has been extensively studied in the laboratory, interactions between stickleback and their predators in nature have never been described. This paper describes interactions between territorial male stickleback and four predators on males or on the young they guard. Two of the predators, cutthroat trout and a hemipteran nymph (Belostomatidae), prey on males but not on young. Another, the roughskin newt, is a potential predator of young but does not pose a threat to males. Prickly sculpin prey both on the males and on offspring that males guard. Differences in behavior toward the four predators indicate that males are capable of rapid discrimination among predators and the threats they pose. Males also discriminate among size classes of sculpin that present different predation risks, but not among size classes of trout large enough to pose a threat. The difference in response to these predators probably reflects the difference in risk to offspring and the amount of time each predator typically stays in the territory of a male. Trout pass through rapidly, whereas sculpin, which are ambush predators, can remain within a territory for a long period of time if not chased out. The presence of young in the nest had no apparent effect on the response of males to these predators. This could, however, be due to a masking effect of uncontrolled variation in natural encounters. Responses to newts were similar to those directed toward sculpin too small to attack the male. They involved rapid chases and bites, often directed at the head, a part of the body that was typically avoided in encounters with larger sculpin. Males watch and avoid the belostomatid nymphs, but do not attack them.