A moccasin snake is the cottonmouth (Agkistrodon piscivorus), a venomous pit viper found across the southeastern United States. It’s the only venomous water snake in North America, which is why it’s also called the water moccasin, swamp moccasin, or black moccasin. The name “cottonmouth” comes from the bright white interior of its mouth, which it displays as a warning when threatened.
How to Recognize a Cottonmouth
Cottonmouths are large, heavy-bodied snakes. Adults typically reach 2 to 4 feet in length, though some grow longer. Their coloring varies with age: younger snakes have bold, dark crossband patterns on a lighter background, while older adults often darken to a nearly uniform olive, brown, or black. This darkening is why they’re sometimes called black moccasins.
The head is distinctly wide and blocky, noticeably broader than the neck. Two features on the head are key for identification. First, cottonmouths have vertical, cat-like pupils rather than round ones. Second, they have a small heat-sensing pit between each nostril and eye, a hallmark of pit vipers that nonvenomous water snakes lack entirely. If you look at a cottonmouth’s head from directly above, the eyes are not visible because of prominent ridges over them. On harmless water snakes, the eyes are clearly visible from above.
Where Cottonmouths Live
Cottonmouths range from southeastern Virginia down through the Carolinas, Georgia, and all of Florida (including the upper Keys), then west across Alabama, Mississippi, Louisiana, and eastern Texas. They also extend northward through Arkansas, southern Missouri, and into the southern edges of Illinois, Indiana, Kentucky, and a small corner of southeastern Kansas. They do not occur in Mexico despite living near the Rio Grande border, and populations that once existed along the Rio Grande in Texas have been wiped out.
Within this range, cottonmouths stick to wetland habitats: swamps, marshes, drainage ditches, lake shores, slow-moving streams, and river floodplains. They’re equally comfortable on land and in water, and they bask on logs, rocks, or banks near the water’s edge. If you’re in the northeastern, midwestern, or western United States and you see a snake in the water, it’s almost certainly a harmless water snake, not a cottonmouth.
Cottonmouth vs. Harmless Water Snakes
This is the question most people actually need answered. Nonvenomous water snakes (genus Nerodia) are common across much of the same range as cottonmouths, and they can look strikingly similar, especially when wet or when they flatten their heads defensively. Here’s how to tell them apart:
- Pupil shape: Cottonmouths have vertical, slit-like pupils. Water snakes have round pupils.
- Heat-sensing pits: Cottonmouths have a visible pit between the eye and nostril on each side of the head. Water snakes do not.
- Head shape from above: On a cottonmouth, bony ridges over the eyes hide them from a top-down view. On water snakes, the eyes are plainly visible from above.
- Swimming posture: Cottonmouths tend to float on the water’s surface with their entire body visible. Water snakes typically swim with most of their body submerged just below the surface. This rule isn’t foolproof, though, since some other nonvenomous species also float.
- Defensive behavior: A cottonmouth will coil, vibrate its tail, and gape its mouth wide open to flash the white lining inside. Water snakes may flatten their heads and strike, but they don’t perform that distinctive open-mouth display.
In practice, the safest approach is to give any snake near water a wide berth and observe from a distance. Cottonmouths are not aggressive by nature. Their threat display is designed to warn you away, not to chase you down.
What Cottonmouths Eat
Cottonmouths are opportunistic predators with a remarkably broad diet. Fish and frogs make up the bulk of what they eat, which makes sense given their aquatic lifestyle. But they also take invertebrates, other reptiles (including smaller snakes), birds, and small mammals. They’re one of the few snakes that will scavenge carrion, feeding on dead fish or other animals they find. This dietary flexibility helps explain why they thrive across such a wide range of wetland environments.
Reproduction
Cottonmouths give birth to live young rather than laying eggs, a trait they share with other pit vipers. Mating typically occurs in spring, and females give birth in late summer or early fall. Litters generally range from about 5 to 12 young, though this varies. The newborns are about 8 to 10 inches long and have a bright yellow or greenish tail tip, which they wiggle to lure small prey like frogs within striking distance. This yellow tip fades as the snake matures.
What Happens if You’re Bitten
Cottonmouth venom is cytotoxic, meaning it primarily destroys tissue at the bite site. It contains enzymes that break down cells and can interfere with blood clotting. The most common symptoms after a bite are pain, swelling, and bruising around the wound, and swelling is the single most frequently reported effect. About 19% of bite victims develop nausea and vomiting, and a similar percentage experience problems with blood clotting.
Not every bite delivers venom. “Dry bites,” where the snake strikes but doesn’t inject venom, do occur. But any bite from a cottonmouth should be treated as a medical emergency because you can’t tell in the moment whether venom was injected. At the hospital, the standard treatment for pit viper bites in the U.S. is an antivenom called CroFab, which halts the spread of tissue damage, reverses blood-clotting problems, and reduces systemic effects. Most cottonmouth and copperhead bite victims respond well to treatment, and only about 2% of Agkistrodon victims (the genus that includes both cottonmouths and copperheads) need large doses of antivenom to bring symptoms under control. Deaths from cottonmouth bites are rare.
Cottonmouth Temperament
Cottonmouths have a reputation for aggression that’s largely undeserved. When threatened, their go-to response is the defensive display: coiling the body, vibrating the tail, and opening the mouth wide to show that bright white lining. This is a bluff, a way of saying “back off” without actually striking. Studies observing cottonmouth behavior have found that they’re far more likely to try to escape or hold their ground with this display than to chase or strike at people.
That said, they will bite if stepped on, grabbed, or cornered. Most bites happen when people accidentally step near one they didn’t see or when someone deliberately tries to handle or kill the snake. If you encounter a cottonmouth, simply moving away is enough. The snake has no interest in following you.