Cottonmouth vs Diamondback Water Snake

The quick tellDiamondback water snakes carry a chain-like net pattern and have round pupils. Cottonmouths are darker and plainer, with a stripe through the eye.

Photo: kirk gardner, CC BY 4.0, via iNaturalist

Cottonmouth

Dangerous · Agkistrodon piscivorus

Heavy, dark olive to black body; bands fade with age
Broad flat head with a dark stripe through the eye
Swims with the whole body high on the water
Opens its mouth wide to show the white lining when threatened

Size: 30–48 in. Habitat: Swamps, slow rivers, marshes and ditches of the Southeast.

A bite is a medical emergency. Most snakes people call water moccasins outside its range are harmless water snakes.

Photo: Susan E Adams, CC BY-SA 2.0, via Wikimedia Commons

Diamondback water snake

Harmless · Nerodia rhombifer

Dark chain-like net pattern on a brown body
Round pupils, no heat pit
Heavy body

Size: 30–48 in. Habitat: Rivers, ponds and swamps of the Mississippi valley and Gulf states.

Not venomous, but bites readily if handled.

Where each one lives

Cottonmouth only
Diamondback water snake only
Both
Neither

Cottonmouth: 15 states. Diamondback water snake: 12 states. Both live in 10 states.

Side by side

	Cottonmouth	Diamondback water snake
Danger	Dangerous	Harmless
Size	30–48 in	30–48 in
States	15	12
Habitat	Swamps, slow rivers, marshes and ditches of the Southeast	Rivers, ponds and swamps of the Mississippi valley and Gulf states

Questions

How can you tell a cottonmouth from a diamondback water snake?

Diamondback water snakes carry a chain-like net pattern and have round pupils. Cottonmouths are darker and plainer, with a stripe through the eye.

Which one is dangerous?

The cottonmouth: dangerous. The diamondback water snake: harmless. If you are not sure, keep your distance and do not handle it.

Where do both live?

Both live in 10 states: Alabama, Arkansas, Illinois, Kentucky, Louisiana, Mississippi, Missouri, Oklahoma, Tennessee and Texas.