"Firewalker" is the ninth episode of the second season of the American science fiction television series The X-Files. It premiered on the Fox network on November 18, 1994. It was written by Howard Gordon, directed by David Nutter, and featured guest appearances by Bradley Whitford, Leland Orser and Shawnee Smith. The episode is a "Monster-of-the-Week" story, unconnected to the series' wider mythology.

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. In the episode, Mulder and Scully investigate a death in a remote research base and discover that a new silicon-based Ophiocordyceps-like fungus found in the area may be killing the researchers.

The plot of the episode was inspired by NASA's Project Dante. "Firewalker" earned a Nielsen household rating of 9, being watched by 8.6 million households in its initial broadcast. The episode received poor reviews from critics, being noted as repeating material already familiar to the series.

Dr. Adam Pierce, a scientist at the California Institute of Technology in Pasadena, intercepts a visual transmission from Firewalker, a mobile robot sent by a volcanic research project on Mount Avalon in the Cascades of Washington. Firewalker is broadcasting from inside a volcanic cave, where Pierce glimpses the dead body of the chief seismologist, Phil Erickson. He also sees a shadow moving in the cave, an impossibility due to the extremely high temperatures. Firewalker's camera is then destroyed, ending the transmission.

Pierce goes to Fox Mulder (David Duchovny) and Dana Scully (Gillian Anderson), showing them a TV interview of the project's leader, Daniel Trepkos (Bradley Whitford); Pierce was with the project until he and Trepkos had a falling-out. Mulder is reluctant to let Scully come along because of her recent abduction; Scully, however, insists that she is ready. Upon arriving at Mount Avalon, Pierce enters the woods to inspect the project's equipment while the agents search the laboratory. They discover the seemingly traumatized team: robotics engineer Jason Ludwig (Leland Orser), systems analyst Peter Tanaka (Hiro Kanagawa), and graduate student Jessie O'Neil (Shawnee Smith). They claim that Trepkos destroyed their lab and disappeared after Firewalker's first descent. Meanwhile, outside, Trepkos strangles Pierce.

After the team finds and stores Pierce's body, Mulder reviews Trepkos' fragmented notes. He finds references to a new silicon-based organism existing inside Mount Avalon; Scully, however, doubts his conclusions because of his mental state. Tanaka breaks into convulsions and displays a high fever, but refuses any help from Mulder or Scully. When he is put on a stretcher for a medivac pickup, Mulder notices a throbbing bulge in his neck. Tanaka flees into the woods and dies when a tentacle-like ascocarp bursts out of his throat. An autopsy by Scully finds sand (silicon dioxide) in his lungs, indicating the existence of a silicon-based lifeform; she theorizes that the organism infects the nearest hosts or otherwise dies. Mulder contacts the CDC to have them quarantine the site.

Mulder and Ludwig venture into the volcanic caves to find Trepkos. When they find Firewalker, Trepkos shoots Ludwig in the back with a flare gun, killing him. He then burns Ludwig's body, as he was infected. Trepkos tells Mulder that after Firewalker returned from its first descent, Erickson pulverized a rock in one of its samples, resulting in his death and the infection of all the scientists immediately surrounding him. Trepkos says that the organism is parasitic in nature, making its hosts pass itself on to others. Meanwhile, in the lab, an infected O'Neil handcuffs herself to Scully to expose her to the fungus, forcing Scully to use a hammer to split the chains but to no avail. Instead, Scully protects herself by throwing O'Neil into a sealed chamber and closing the door. The fruiting body breaks out from O'Neil's throat, killing her without spreading the disease any further.

Mulder and Trepkos arrive on the scene. Mulder radios the evacuation team but—knowing that Trepkos will refuse to go—reports that only he and Scully have survived the ordeal. The agents enter a month-long quarantine while the Chemical Corps confiscates the lab and cordons off Mount Avalon. Firewalker is salvaged, but is too damaged to yield sufficient data. Trepkos and O'Neil are officially unaccounted for and presumed dead; Trepkos is last seen carrying O'Neil's body as he disappears into the volcano.