Evolutionary information.

Like most Pokémon with additional evolutionary forms, Bulbasaur evolves once it reaches a certain level. In most cases, with the exception of player trades, Bulbasaur is received at level 5. As soon as it hits level 16, Bulbasaur will evolve into Ivysaur. Most starter Pokémon evolve at level 16 into their secondary forms, with few exceptions. Additionally, their secondary forms usually evolve into their tertiary forms at level 36. Ivysaur, however, will evolve instead at level 32 into Venusaur.

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Mega-evolution.

Bulbasaur's final evolutionary form Venusaur is one of a select few Pokémon given a new, albeit temporary, evolution in Generation VI. With the introduction of Mega-Evolution in the Pokémon X and Y games, players were able to select one of the three original starters. Once chosen, they would also receive a special item that would give that Pokémon's final evolution the ability to Mega-Evolve in the midst of battle. This new evolution can change types, stats, and abilities. In Venusaur's case, whatever it's original ability is, it will be changed to Thick Fat, which allows 1/2 damage for Fire and Ice-type moves, two of this Pokémon's type weaknesses. In addition, Venusaur will receive an additional 100 points added onto it's base stats.