This is the first American film made by the great French director Jean Renoir. It was filmed in the studio and also on location in the Okeefenokee swamp. A title in the very beginning tells us that the swamp in those days was hundreds of miles of unknown wilderness filled with alligators and "the deadly cotton-mouth snake." In the movie everyone is scared to go too far into the swamp.
A young man named Thursday who was part of a search party looking for two lost trappers, who we later find out were "cottonmouth bit" swelled up and died. After his dog runs out of the boat after a deer into the swamp he returned to town leaving it there. Then he decides, against the advice of everyone in town, to go back into the swamp alone to look for the dog. He finds it living with a fugitive played by Walter Brennan. Brennan is wanted for a murder he didn't commit, and his daughter, a wild girl nicely played by Anne Baxter, is living in town and later gets involved with Thursday.
Brennan goes to a pool of water near his camp at night to wash his face and a cottonmouth strikes him in the face. He passes out so Thursday digs a grave for him only to return to their camp to find him alive and sitting up. Brennan tells him that he's been cottonmouth bit a dozen times - he just makes his mind up not to die and he doesn't.
This movie was remade in color in 1952 as "Lure of the Wilderness." It also stars Walter Brennan. It has two more snake appearances but otherwise it's nearly identical. An interview with the snake director of that movie describes that a real Cottonmouth was filmed separately from Brennan, then the two films were merged. It looks like the same technique was used here, although the lighting is so dark, it's hard to even see the snake strike.