Marshtomp debuts as a Mudkip in the episode A Mudkip Mission and it was the only Pokemon to protect the Mudkip babies coming from Swampy. Brock uses Lotad to save the baby but it was saved by Mudkip itself. Brock wanted to befriends it by feeding his Pokemon food but it was interuppted by Ash and the group causing Mudkip to run away. When Team Rocket attempts to steal all the Mudkip babies, Mudkip teams up with Lotad using Water Gun to free them. Mudkip is the only Pokémon to save the last baby from a current until it hangs on the waterfall. Brock eventually saves it from falling.
After the incident, Swampy allows Brock to keep Mudkip due to its liking and has no friends to take care of it. Brock finally received Mudkip to join the group.
Mudkip is used to settle feuds throughout the series. In A Shroomish Skirmish, it teams up with Pikachu to stop both Corphish and Torchic from fighting but failed and Ash's Grovyle is the one to stop then from fighting by separating them with a Leaf Blade. After being separated by a wild Breloom's Mach Punch, Mudkip goes with May to search for the others until it saves Max, Meowth, Pikachu, Torchic and an injured Corphish from Breloom's attack with Water Gun. It also witnesses with Pikachu, Meowth and Corphish to see Torchic angrily evolves into a Combusken to fight Breloom which its leader.
In A Chip Off The Old Brock, Mudkip spars with Ash's Grovyle until it evolves into a Marshtomp as it attempts to hit it with its newly learned Mud Shot but failed and it accidentally hits Mariah's Flaaffy. When trying to save both Mariah and McCauley from Team Rocket, it was used to save them but failed and it accident hits Pikachu and Katie's Mareep forcing them to get cured from their medicine. After the incident, McCauley and Mariah decided to have a new medicine shop leaving Brock and Marshtomp got rejected. In the early events of the episode, May even comments that Marshtomp becomes a Junior version of Brock.
After evolving, Brock's Marshtomp became a strong Pokémon often used to settle feuds or in simple battles. Marshtomp is a well mannered Pokémon which takes after its trainer a little bit in regards to romance. Brock notably used Marshtomp in a Pokémon Contest against May.
|Water Gun||A Mudkip Mission|
|Mud Shot||A Chip Off the Old Brock|
|Protect||What I Did For Love!|
|Tackle||What I Did For Love!|
|+ indicates this Pokémon used this move recently.*|
- indicates this Pokémon normally can't use this move.
- Megumi Hayashibara (Japanese as a Mudkip)
- Shin-ichiro Miki (Japanese as a Marshtomp)
- Lindsey Warner (English 4Kids dubbed as a Mudkip)
- Michele Knotz (English PUSA dubbed as a Mudkip)
- Kayzie Rogers (English as a Marshtomp)
- Brock's Marshtomp is the only Hoenn Starter Pokémon that isn't voiced by Darren Dunstan and later Bill Rogers in its first stage evolution.
- Marshtomp is the only starter Pokémon owned by Brock.
- Marshtomp is the only Hoenn starter Pokémon owned by a main character that didn't reach its final evolution.