Agkistrodon contortrix phaeogaster was formerly a venomous pit viper subspecies found in the central region of the United States. However, recent taxonomic changes do not recognizes the Osage copperhead (A. c. phaeogaster ) as a valid taxon.
Gloyd and Conant (1990) gives maximum total lengths of 990 to 723 mm for males and females, respectively, while mentioning that other sources give a maximum total length for this subspecies of 1,016 mm regardless of sex.
The color pattern is similar to that of A. c. mokasen, except that the dark bands are in sharper contrast to the lighter ground color, and with no smaller dark spots between them.
Found in the United States in eastern Kansas, extreme southeastern Nebraska and a large part of Missouri. The type locality given is "10 miles south of McLouth, Jefferson County, Kansas."
The venom of Agkistrodon contortrix phaeogaster is similar to the venoms produced by other species of the Agkistrodon genus. The exact ratio of toxins to each other varies among species and gender, as does the overall potency of the venom (the ratio of active toxins to other constituents of the venom). The venom is most potent between the late-spring and mid-summer months, this rise in toxicity commences 2–3 weeks prior to reaching peak toxicity. While the initial reduction of potency occurs over approximately one month, the snake's venom remains slightly more toxic throughout the late-summer months until beginning a slow descent to reach its original toxicity by mid-fall.
