"The Pine Bluff Variant" is the eighteenth episode of the fifth season of the American science fiction television series The X-Files.

"The Pine Bluff Variant" received a Nielsen household rating of 11.4 and was watched by 18.24 million viewers in its initial broadcast.

The show centers on FBI special agents Fox Mulder (David Duchovny) and Dana Scully (Gillian Anderson) who work on cases linked to the paranormal, called X-Files.

It is eventually revealed that Mulder is working as a mole in the group, and he is trying to stop them before they are able to use a biological weapon—that may have been created by members of the U.S. government—which causes rapid degeneration of human flesh.

"The Pine Bluff Variant" was based on the idea of Mulder going undercover, a topic that Shiban had wanted to work on for a majority of the fifth season.

The title is also a reference to the Pine Bluff Arsenal, a real United States military base with stockpiles of chemical weapons.

Fox Mulder (David Duchovny), Dana Scully (Gillian Anderson), Walter Skinner (Mitch Pileggi), and several other FBI agents hide in a Washington park to catch Jacob Haley (Daniel von Bargen), the second-in-command of an anti-government militia called the New Spartans.

[2] According to Shiban, the 1995 action film Heat was an inspiration for the episode,[3] as was the 1963 novel The Spy Who Came in from the Cold, written by John le Carré.

[2] The writing staff also played off then-recent fears about Saddam Hussein's use of biological weapons to craft the basic premise of the script.

Von Bargen was chosen due to his experience in several films, whereas MacRae had previously been cast in the first season episode "The Jersey Devil".

[5] The movie that is being shown is Die Hard with a Vengeance (1995), whose storyline of a mission by terrorists involving American banks is broadly mirrored in this episode's plot.

[8] He called it "an excellent episode" that is "notable for its tension" and the fact that it "seemingly tells a story that has little to do with the X-Files or Mulder and Scully's search for the truth—right up until the final twist.

"[8] Furthermore, Handlen called Shiban's script "by far his best" and noted that the melted corpses in the episode provided "a striking, deeply creepy visual".

[8] Robert Shearman and Lars Pearson, in their book Wanting to Believe: A Critical Guide to The X-Files, Millennium & The Lone Gunmen, rated the episode four stars out of five.

[9] The two called the entry "cleverer than most" latter season episodes that feature Mulder or Scully undergoing a psychotic break, due to its "straight-forward thriller" sensibilities.

[9] Jonathan Dunn, writing for What Culture, highlighted "The Pine Bluff Variant" for its cinematic appeal and included it in the "5 Episodes [of The X-Files] That Could Be Made Into Movies" list.