Could Mulder be a 'J'... He is single minded like INTJs can get, but I think he's too 'chaotic' in his thinking process to be a J. I lean towards he being an, so to speak, 'unhealthy' INFP too, devoted to something very personal. I would say Scully is an SJ, as she hardly ever argues with Mulder on his level, she usually tries to bring him to the realms of tangible reality and facts.
I can't see the NF a lot of people see in Mulder. Yes, he did become obsessed with his work because of his sister's obduction, but INTP's can be very sentimental and human too . He isn't searching for some mystical truth- he's trying to fully understand the truth that is obscured from him- something real that he just can't see or can't see yet. He is extremely autonomous and operates calmly in intense emergencies. Interpersonal thought is not at the forefront of his psyche. His "trust no one" code and ability to see through people who may be untrustworthy by analyzing the situation leads me to strongly believe that he is an INTP. Also, he appears to extrovert intuition.
Scully has a hard time entertaining the visions and hunches that Mulder's Ne produces. If she didn't have to work so close to him, she probably wouldn't entertain his hunches at all; they simply wouldn't occur to her. She is so comfortable with "just the facts" and a complete workhorse. Though she comes off as an NT in some ways, I am drawn to believe that she is an ISTJ rather than an INTJ for various reasons including her complete absence of taking interest in abstract thinking like someone else mentioned.
So, I am in agreement with the majority here in thinking that Mulder is an INTP and Scully an ISTJ.
"How dreadful!" cried Lord Henry. "I can stand brute force, but brute reason is quite unbearable. There is something unfair about its use. It is hitting below the intellect." ~ Oscar Wilde - The picture of Dorian Gray