Agkistrodon contortrix mokasen (or northern copperhead) was formerly a venomous pit viper subspecies[2] found in the eastern United States.

The resulting taxonomy does not recognize the northern copperhead (Agkistrodon contortrix mokasen) as a valid taxon.

Juvenile specimens are lighter in color, and have a yellow tail tip and a narrow dark line that runs through the eye that divides the darker head from the lighter-colored labial scales.

[8] This subspecies is found in the United States in Washington D.C., Maryland, Virginia, East Texas, southern Illinois, southern Indiana, throughout Mississippi,[11] northern Alabama, northern Georgia, northeast to Massachusetts (which considers them endangered), New York Hudson Valley region, the Appalachian Mountain region and associated plateaus, and southwestern Pennsylvania.

[1] These snakes are generally quiet, almost lethargic, preferring to lie motionless or to make a slow retreat when encountered.