Bull snake - aka Gopher snake (Pituophis catenifer).
A type of constrictor.
The gopher or bull snake can reach a length of 7 feet (it is Montana's largest snake), but most range from 3 to 5 feet. It can be readily identified by a series of large black to brown blotches that run down the back, and another series along the sides. The blotches, which are set on a yellow background, become more widely spaced toward the tail. The dorsal scales are keeled (have a ridge running down the center). A black band can usually be seen on the head in front of and extending below the eyes. The belly is yellow to white, often spotted with black.