Jumping spiders (Salticidae) usually avoid ants, but some species within this family single out ants as preferred prey, while others (especially the species in the genus Myrmarachne) are Batesian mimics of ants. Field records show that ant-eating salticids sometimes prey on Myrmarachne, suggesting that the unwanted attention of predators that specialize on the model may be an important, but poorly understood, cost of Batesian mimicry. By staging encounters in the laboratory between living ant-eating salticids and Myrmarachne, we determined that ant-eating salticids attack Myrmarachne. However, when Myrmarachne detects a stalking ant-eating salticid early enough, it adopts a distinctive display posture (legs almost fully extended, elevated 45°, and held out to the side 45°), and this usually deters the predator. When Myrmarachne detects an ant-eating salticid before stalking begins, Myrmarachne makes preemptive displays that appear to inhibit the initiation of stalking. Using immobile lures made from dead Myrmarachne that were either in a display posture or a nondisplay posture, we ascertained that specifically the display posture of Myrmarachne deters the initiation of stalking (ant-eating salticids stalked nondisplaying more often than displaying lures). In another experiment, we ascertained that it is specifically the interjection of display posture that deters stalking. When ant-eating salticids that had already begun stalking experienced lures that switched from a nondisplay to a display posture, they stopped stalking. Although the unwanted attentions of its models' predators may be, for Myrmarachne, a hidden cost of Batesian mimicry, Myrmarachne appears to have an effective defense against these predators. © The Author 2006. Published by Oxford University Press on behalf of the International Society for Behavioral Ecology. All rights reserved.