The Cerulean Gym is designed like an indoor swimming pool, with platforms above the water for the player to walk on. There are no puzzles to solve in this gym. A field based on the gym also appears in Pokémon Stadium's Gym Leader Castle.
Trainers with a telephone symbol by their names will give their Pokégear number to the player, and may call or be called for a rematch with higher-level Pokémon.
Trainers with a PokéNav by their names will be registered in the Trainer's Eyes or Match Call function after the first battle, and may have a rematch with the player with higher-level Pokémon.
Cerulean Gym has appeared in several episodes of the anime. On the exterior, the building is a dome-shaped brightly colored Gym with a gigantic painting of a Dewgong on top of it. On the inside, the gym is equipped with a large aquarium and swimming pool.
It first appeared in The Water Flowers of Cerulean City. Ash was heading there so he could earn his Cascade Badge. Misty kept telling him not to go to Cerulean, and he soon discovered why she tried to stop him; her three sisters, Violet, Lily and Daisy, were the gym leaders. However, they had turned the gym into a place for them to put on their water acrobatics shows. They were about to give Ash the badge when Misty intervened. She challenged Ash to a battle in her capacity as an official Gym Leader, but it was interrupted by Team Rocket. In the end, Daisy gave Ash the Cascade Badge, saying that if it weren't for him, the gym would have been destroyed.
The gym's next appearance was in The Misty Mermaid. While on the way to the Viridian Gym, Misty's Horsea had become sick. Realizing the gym was nearby, Misty decided to go and see if they could make Horsea feel better. The Dragon Pokémon felt better immediately. However, the sisters made Misty participate in one of their water ballet shows. During a Team Rocket battle, the Seel the Sensational Sisters had in their first appearance evolved into Dewgong and saved the gym. Before leaving, Misty left her Horsea and Starmie at the gym, hoping one day to come back for them.
