Expanded Trading
Trading between different versions of the Pokemon games enables players to keep a wider range of Pokemon, since certain types of Pokemon are rare in each version. "Pokemon Fire Red" is automatically able to trade with "Pokemon Leaf Green," but to unlock trading with "Ruby" and "Sapphire" versions, collect the Sapphire and Ruby in game items. Go back to the first island, then go to the Network Machine. Select the stones to fix the machine, then, save the game. Connection between "Ruby" and "Sapphire" versions will now be possible.
Exclusive Pokemon
Some Pokemon are missing from the "Fire Red" version and must be traded from "Leaf Green," "Ruby" or "Sapphire" versions. These Pokemon include Sandshrew, Sandslash, Vulpix, Ninetales, BellSprout, WeepingBell, Victreebell, Slowpoke, Slowbro, Slowking, Staryu, Starmie, Magmar, Magby, Pinsir, Azurill, Marill, Azumarill Mistreavious, Sneasel, Remoraid, Octillery and Mantine. Additionally, collecting all Pokemon (including these "trade-only" types), will complete your National PokeDex and unlock the Diploma achievement.
Action Replay Codes
For players with an Action Replay cartridge for their Game Boy Advance, codes can be used to trigger which type of Pokemon you will encounter next. This can be used to control which Pokemon you meet -- and which Pokemon you catch -- to help players expand their Pokemon collection. To encounter a Fire Type Pokemon, enter 17543C48 E65E0B97; for a Leaf Type Pokemon, enter 594F7021 3B4E81A7. Specific Pokemon can also be triggered -- enter AD86124F 2823D8DA for Bulbasaur, DB8C60CD 81FD6AA7 for Charmander, BCDCB406 2F2FD763 for Squirtle or 9D4A1BFF 05120D39 for Pikachu.
Action Replay Specialty Codes
To encounter more rare Pokemon, the following codes can also be used with an Action Replay cartridge: for Scizor, use F8D4BFAE D2C136F5, or for Entei, use 0C8037A6 BE1D9DB2. D7B594AF C1DA18AB will unlock Articuno, while 83E25054 9C927E5E and F8DEC3CC A014B094 will unlock Zapdos and Moltres respectively. For Mewtwo and Mew, use 4C77BA3F CA4EDCB6 and B751BDF4 95CEF4CC.