Yakouchuu is an adventure game. The title translates to Phosphorescent Animalcule (animalcule meaning a microscopic animal, a portmanteau of animal and molecule) and specifically to Noctiluca scintillans.
It is a `sound novel` in the vein of Chunsoft's hits Otogirisou and Kamaitachi no Yoru (localized as Banshee's Last Cry): the player reads a novel-like story as the text appears on the screen with a static backdrop and background sounds to set the scene. They also occasionally make choices which can determine the route of the story. Yakouchuu concerns the ship `Diana` and its captain, who is the player-named protagonist, and crew as it sets off on an inauspicious journey.
The game was ported to the Game Boy Color in 1999. The same year, it saw a sequel for the N64: Yakouchuu II: Satsujin Kouru.