Escape behavior of the natricine snake Nerodia sipedon when approached by a human was studied in populations in wetlands in Ohio and Michigan. Snakes responded by dropping from vegetation, but not immediately fleeing (44%), by diving beneath the water (37%), or by swimming away with the head above the surface (19%). Flight initiation distance (distance between predator and prey when escape begins) was significantly greater for individuals that dropped than those that dove or swam on the surface. This novel finding suggests that dropping may position snakes well to escape should approach continue and perhaps might serve as a pursuit-deterrent signal. Adult females were warier (had greater flight initiation distance) than adult males or juveniles. Several hypotheses to account for this difference are discussed. Flight initiation distance increased with starting distance (distance between predator and prey when the predator begins to approach). This effect was much stronger than in the few lizard species studied. We propose that degree of risk assessed by snakes may change during approaches because a predator that moves directly toward a snake for a longer distance is more likely to have detected the snake and be attacking than a predator that approaches over a shorter distance. Increase in perceived risk would account for greater flight initiation distance associated with greater starting distance. Flight initiation distance for basking snakes did not vary with air temperature or perch height, contrary to previous findings for body temperature and perch height. Reasons for these findings are discussed.