Dr. Leslie Arzt was a middle section survivor of Oceanic Flight 815. He was a science teacher who managed to discover many new species of insects and arachnids, including the Medusa spider, but was killed on the Island while handling dynamite on Day 44.
Before the crash
Leslie Arzt was a ninth-grade science teacher who had been married three times. (1x24). Sometime before the crash, he had met a woman on the Internet using a picture of his buddy Nick. They arranged to meet in Sydney, but the woman disappeared after he had ordered lobster in a restaurant. Arzt decided not to go out into the Sydney nightlife, but to book an early flight home instead. This turned out to be Oceanic Flight 815. (mx09)
On the Island
When Jack wanted all the survivors to move to the caves, Arzt expressed his doubts that it was a good idea. He told some of the other survivors, including Hurley and Michael, that he doubted Jack's ability to lead them. Just then everyone heard the monster, and Arzt changed his mind, deciding he would be safer in the caves than he would on the beach. (mx07)
Arzt had a small structure built for himself where he kept a large collection of island insects which he said were previously undiscovered species. When approached by Nikki, Arzt told her about a dangerous spider and at her request, helped her figure out where her luggage could have potentially been thrown from the plane. He ineffectually flirted with Nikki and comically tried to impress her. (3x14)
A few days later, Arzt and Shannon argued with Kate because the guns from the Marshal's case were kept secret. (3x14)
Days 43-44 (Season 1)
Arzt warned Michael that he shouldn't wait to set sail on the raft because of the trade winds shifting south. He used a windsock to demonstrate them, claiming he knew all of this because of his profession. Sawyer questioned why they should listen to a science teacher and Arzt replies, "Because I'm a doctor, and you're a hillbilly." (1x22)
However, Arzt later revealed to Michael he had made the whole monsoon season story up just to get Michael to launch the raft as soon as possible. He apologized for lying and expressed his faith in them being able to bring back help, saying that he'd be the first to greet them when they came back. (mx09)
When Locke suggested hiding survivors inside the hatch, Jack and a group of Losties set out on a mission to the Black Rock to retrieve dynamite to blow open the Hatch door. Arzt volunteered to go along with them, saying he knew "how to handle volatile dynamite better than anyone else." (1x23)
Danielle Rousseau led the group into the woods and abandoned them once they had reached the old slave ship. Jack, Kate and Locke found a crate of old and very unstable dynamite inside the vessel and brought it to Arzt, who, ironically, triggered one of the sticks and blew himself up while lecturing the others on how to handle it. Pieces of his body scattered throughout the area. The rest of the group took this as a warning as they continued to wrap the dynamite in wet cloth. When Jack and Locke pull out the dynamite from the box, a shaken Hurley points to something on Jack's back and says "You have some...Arnzt...on you." (1x24)
Flash-sideways Timeline (Season 6)
Aboard Flight 815
Arzt recognizes Hugo from his commercials for Mr. Cluck's Chicken. He begs him to do one of the voices from his commercials, and laughs hysterically when Hugo finally gives in. He excitedly tells another passenger that Hugo is the owner of Mr. Cluck's and is impressed that he is "riding coach with the rest of us yutzes," but the other passenger seems unimpressed. Arzt then asks how a guy like Hugo was able to buy a major corporation, and Hugo simply explains that he won the lottery and liked chicken, so he bought it. Arzt seems disappointed and slightly irritated with this answer and mutters "good for you" before walking away. (6x01)
After the Flight
Later, Arzt walks in front of Kate's cab and has some of his luggage run over when Kate makes the driver continue driving (What Kate Does).
- While on the way to the Template:Link, Hurley had trouble pronouncing Leslie Arzt's last name, which is German for "doctor of medicine" or "physician" (der Arzt, die Ärzte). Here is their exchange, which was retrieved from the episode transcript:
HURLEY: Let me ask you something, Arnzt --
LESLIE ARZT: Arzt.
LESLIE ARZT: No, not Arnzt. Arzt. A-R-Z-T. Arzt.
HURLEY: Sorry, man. Your name's hard to pronounce.
LESLIE ARZT: Oh, yeah? Well, I know a bunch of ninth graders who pronounce it just fine.
HURLEY: How about I just call you by your first name?
LESLIE ARZT: How about you don't?
HURLEY: Why not? I remember it from the plane's manifest. I think Leslie's a bitchin' name.
LESLIE ARZT: Arnzt is fine.
- On the ABC.com website section "Lost Connections", Arzt's name is spelled improperly as "Artz."
- Charlie also mis-pronounced his name as "Artz" multiple times throughout season two.
- ABC misspelled his name (again, "Artz") in the press release for "". 
- The title of the mobisode "" was a humorous allusion to this name confusion.
- ABC misspelled his name (again, "Artz") on the Lost but not forgotten video (00:14) during Comic Con 2009
Action figure biography
On the back of the box for the Highly Flammable Toys Arzt custom-made action figure, there is a brief biography. According to Daniel Roebuck, this information was written by Javier Grillo-Marxuach, and can therefore be reasonably considered to be canon.
Born in an unremarkable suburb of Jersey City, Leslie Arzt soon demonstrated the kind of promise that could only herald a doctorate from an unremarkable university in the state system. Though not a particularly patient or didactic man, Leslie Arzt soon parlayed the staggering lack of interest shown him by employers in the private sector into a long-term career teaching science in the New Jersey Public Schools system. While neither a student nor faculty favorite - and in brazen defiance of brusque personal style that rendered him unfit for the job - Dr. Arzt (as he insisted on being called by everyone from students to school board administrators) courageously expanded his activities from mere teaching into teachers' union politics. After giving decades of his life and sacrificing - by his own account - three marriages to his activities, Dr. Arzt finally took a vacation to Sydney, Australia. The cataclysmic crash of Oceanic 815 left Dr. Arzt one of 48 survivors stranded on an uncharted island. On the island, Dr. Arzt soon went about making friends and influencing people in his own indomitable style - alas, this bright light was snuffed out explosively during an impromptu lecture on the instability of nitroglycerin. Few who knew him will truly miss him.
- Out of all the main characters, Arzt only met the Season 1 cast along with Nikki and Paulo.
- There is a fake warning included with the Arzt action figure: 'For 9th graders on up'. (See above)
- Leslie is of Scottish origin, and its meaning is "meadowlands".
- Arzt is German for Doctor, hence his name was Doctor Leslie Physician.
- Arzt may have had a Ph.D as Sawyer, Charlie and Nikki and Arzt himself all referred to him as Dr. Arzt.
- Daniel Roebuck, the actor who portrayed Arzt, was also in the film "The Fugitive", in which he co-starred with L. Scott Caldwell, who played Rose. The two were finally featured in an episode together, during season 6.
- Arzt mentions being a teacher at Pali, an homage to creator J.J. Abrams' high school.
- During Comic Con 2009, a video was shown in memory of significant characters who have died on Lost. Arzt's name was spelled Artz which is incorrect.