Mulder's address is #42, 2630 Hegal Place, Alexandria, VA 23242 ("Small Potatoes" and "Monday"). However, 23242 is a ZIP code in the vicinity of Richmond, Virginia. He was living at that address at least as far back as 1990 ("Travelers").
Mulder's apartment number, 42, is a reference to the answer to life, the universe and everything. Scully apparently has a key ("End Game", others), although when she acquired it is a mystery.
Mulder's badge number is JTT047101111 ("F. Emasculata")
Mulder's computer password is TRUSTNO1. He uses computer software that displays the password as it is typed ("Little Green Men").
Pseudonyms Mulder has used include M.F. Luder, George E. Hale, Marty and Mr. Kaplan ("The Pine Bluff Variant"). Jose Chung refers to him as "Reynard Muldrake" in his book, with "reynard" being the antiquated spelling for "fox" in French ("Jose Chung's 'From Outer Space'").
Mulder has a tank with goldfish at home that Scully feeds in his absence.
He hates being called Fox, and claims that he even made his parents call him Mulder growing up. However, both parents call him Fox on the show.
The character of Fox Mulder has also appeared on other series - The Lone Gunmen and Millennium. He was mentioned in the short-lived series Strange Luck. He also made an appearance in the "The Springfield Files" episode of The Simpsons and on the animated series Eek! The Cat. In an episode of the ReBoot series, Mulder and Scully were parodied as agents "Fax Modem" and "Data Nully." David Duchovny refused to provide his voice for the episode.
He is possibly of partial Dutch heritage, as Mulder is a Dutch surname meaning Miller. Chris Carter has said that he named Mulder after his mother's maiden name. His first name, Fox, was actually not a tribute to the FOX network which aired The X-Files, as often assumed. Carter said he had a childhood friend named Fox.[10]
According to the 5th season episode "Unusual Suspects", which took place entirely in 1989, Mulder is 6' tall (~183cm), about 170lb (~77kg), and has green eyes.
In the Eposode "Hollywood AD" Mulder states that has seen Plan 9 from Outer Space 42 times (another reference to The Hitchhiker's Guide to the Galaxy), and claims that the film is so incredibly bad that it shuts down the logic centers of his brain, allowing him to make intuitive leaps of logic. He called this the "Ed Wood Investigative Method".
According to the episode "Beyond the Sea," Mulder's blood type is O-negative.
In the episode "Wetwired", he states that he is red-green colorblind, although this trait does not appear to interfere with his casework or personal life, and indeed is never mentioned anywhere else in the show.
In the episode "Je Souhaite", while watching Caddyshack at Mulder's apartment, Mulder and Scully are shown drinking Shiner Bock beer.
Mulder's favorite snack is sunflower seeds. In "Aubrey" (Episode #36), he reveals to Scully that his father used to eat them as well and suggests that his taste for sunflower seeds may be some sort of genetic memory. In "Anasazi" (Episode #49), Mulder asks for seeds while delirious with a fever. The audience first sees him eating them in "Squeeze" (Episode #03), Scully is on a stakeout and Mulder offers her some seeds from the bag he is holding.
In episode 3X04, "Clyde Bruckman's Final Repose", Clyde suggests that Mulder will die from Autoerotic Asphyxiation.
In season 6 episode "The Unnatural", we find that Mulder is a fan of baseball.
In the videogame Ape Escape 2 for the Playstation 2, there is an ape named Mulder with the tagline "The truth is out there!".