Growlithe is in a small group of Pokémon that do not evolve normally by "leveling up." Instead, this Pokémon must evolve by using a Fire Stone.
Because of the nature of Growlithe's evolution into Arcanine, it can evolve at any level. The player must use some type of strategy in determining when to evolve it, since this Pokémon is unable to learn most of its moves after evolving. In general, it is recommended to wait until at least level 45 to evolve it so it can learn Flare Blitz, the most powerful move that its evolved form, Arcanine, cannot learn. However, Arcanine has a higher growth rate in its stats, so evolving Growlithe earlier than level 45 may produce a more powerful Pokémon.