A semiaquatic pit viper (subfamily Crotalinae), up to 1.2 m long with a thick body and large, triangular head. It is olive with brown or yellow bands when young, turning to dark brown or black as it ages. A cottonmouth swims well, with its head held above the surface. When threatened, it coils its body and opens its mouth wide, revealing the white interior. It rarely bites people, but its bite can be fatal. It occurs in the south-eastern USA.