Jungle, as often dubbed by the fans (after the level of the same name) is one of the most popular and memorable, if not the most, background tunes on the Contra series. It, along with the rest of the soundtrack to the first Contra game, was written by Hidenori Maezawa. It's first appareance was in the first incarnation of the game, Contra, and has since then appeared multiple times in other games of the series, in the form of remixes or remakes.
Original Contra - First level.
Contra 4 - First level, if the player chooses the Hard difficulty.
Contra: Shattered Soldier - Fifth level.
Contra Rebirth - First level
Hard Corps: Uprising - The first level's song starts with a riff taken from the original Jungle song. If one enters the Konami Code in the loading screen for this level, a remake of the song is heard instead.