There are two codes that are used to complete the Pokedex and enable the National Dex in "Pokemon LeafGreen." The complete Pokedex code is 4202462CFFFF, 0000003C0002, 42025BA0FFFF, 0000001A0002, 42028FC0FFFF, 0000001A0002. The code that enables the National Dex is 3202461F00B9, 320265900001, 820266446258.
The code to complete the Pokedex should be entered before the National Dex code. These codes show all 386 Pokémon as seen in the player's Pokedex. The codes do not show the Pokémon as captured. The National Dex is unlocked after he defeats the Elite Four and completes the Sevii Islands sidequest. This enables the player to trade Pokémon with previously released games.