"The Beast Within" is a very loose adaptation of Edward Levy's 1981 novel of the same name, with the plot centering on a couple's son, who begins exhibiting strange behavior after his 17th birthday. Producer Harvey Bernhard had purchased the rights to Levy's then-unfinished novel based on the title alone, but the resulting screenplay bears little resemblance to the novel as it was still incomplete at the time of production. Principal photography officially began on February 8, 1981, in and around Jackson, Mississippi and concluded on April 10, 1981.