Sex differences in the behavior of the highly predatory northern grasshopper mouse (Onychomys leucogaster) were investigated in a series of eight experiments. Female mice acquired two-way avoidance behavior more slowly, but passive avoidance more rapidly, than males. Females also buried a shock probe more completely than males and showed more escape behavior during open field testing. There were no sex differences in flinch-jump thresholds or escape from electric shock, general activity levels, or acquisition and reversal of a position discrimination. It is suggested that the sex differences in aversively motivated behavior result from an increased tendency of females to respond defensively to focal sources of aversive stimuli. The enhanced defensiveness of females may be an adaptation to higher levels of pup predation resulting from the large territories required by their predatory specialization and their association with other, more abundant, rodent species.