Prey avoid predators in a wide variety of ways. Grasshoppers of the alpine habitat exhibit predator-avoidance posturing. Furthermore, they show sexual diethism (behavioural differences between sexes) in which the males of Aeropedellus clavatus (Thomas) and Melanoplus marshalli marshalli (Thomas) prance (hop without progression), tending to face the oncoming threat, more so than the females. The latter do not prance to the same extent and more frequently face away from the threat. On the plains, Melanoplus bivitattus (Say) and M. sanguinipes sanguinipes (Fabricius) tend to assume a sideways stance relative to the oncoming threat. Neither plains species exhibits sexual diethism in avoidance behaviour. Prancing occurs in the plains species but, in males, is less frequent than in alpine species. These different predator-avoidance behaviours are apparently related to the need for the grasshoppers to maintain surveillance of potential predators in habitats of different temperature regimes and density and height of cover.