How to Catch Mew in Pokémon Red/Blue/Yellow

Yes, believe it or not, it can be done without a GameShark or other cheating device.  Here's what you'll need to do:


 
Make sure you have a Pokémon that knows Fly.  On Route 8, stand in front of the door of the Underground Path.  Save your game, since saving after this point will disrupt the procedure.
Walk down one step and immediately press Start.  If you succeed, the menu will pop up before the Gambler "sees" you. Go to the Pokémon list and fly to Cerulean City.
The Gambler will see you just before you fly away.
Your Start button will no longer function.  Go north across Nugget Bridge and head east until you find the trainer in this picture (he's the Youngster described above).
If you walk right up to the Youngster, the game will lock up, so you'll have to let him see you and walk up to you.  Defeat his Slowpoke (afterwards, your Start button will work again), then fly to Lavender Town.
Head west into Route 8.
As soon as you enter the narrow path, the menu will pop up by itself.  Press B to exit...
...and look who it is!  Mew will be at Level 7 and its only attack will be Pound.  Pokéball, go!

Once you've caught Mew, you can save your game without any ill-effects.

There are two alternative ways to enter Route 8 and trigger Mew after beating the Youngster.  If you have access to Saffron City, you can fly there and go through the eastern city gate.  Also, you can fly to Celadon City, head east to Route 7, enter the Underground Path, and emerge back in front of the Gambler.

Finally, the Route 8 Gambler isn't the only trainer that can initiate the trick.  The Super Nerd just west of him can also do it, as can the second Fisherman you meet when heading south from Lavender on Route 12.  There are numerous others; basically, any trainer who is scrolled onto the screen by the same step that puts you in their vision range will work.  However, remember that the final battle with Mew will only be triggered by returning to whatever route that trainer is in, which is not necessarily Route 8.

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How the Mew Trick Works

If you're not a geek, you don't wank to read this. :)

When you use Fly to get away from the Gambler, the game somehow "remembers" that a battle is supposed to be taking place on Route 8.  This is what causes your Start button to stop working.  The rest of the trick hinges on a twelve-byte section of memory, starting at memory location CD22 in Yellow (CD23 in Red/Blue), which contains the stats of the last Pokémon you fought.  After you battle the Youngster, it will have these hexadecimal values:

The first byte (8E) seems to be the same for all battles.
The second byte (11) is the enemy's level (17 in decimal).
The next ten bytes are the enemy's stats (HP/Attack/Defense/Speed/Special).  Each stat takes up two bytes.
Additionally, this section of memory is surrounded by eight bytes on each side, which all have the value 07.

The last byte (15) in Yellow memory location CD2D (CD2E in Red/Blue) is the important one.  It is the "least significant byte" of the enemy Pokémon's Special stat; in this case, it means that the enemy has a Special of 21 in standard decimal notation.

When you return to Route 8, the game tries to resume the battle that was interrupted before.  For reasons unknown, it borrows byte CD2D to use as the species identifier for the enemy Pokémon, and the next byte to use as its level.  If the last battle you fought was with the Youngster, these bytes will be 15 and 07.  Since Mew's hexadecimal identifier is 15, you will fight a Level 7 Mew.

How to Catch Any Pokémon

Now that you know how the trick works, you can use it to catch other Pokémon.  Fighting the Youngster will always result in a Mew encounter because his Slowpoke has a Special of 21 (15 in hex).  But if you fight something else before returning to Route 8, it will change which Pokémon you encounter.

Let's say you foolishly choose a starting Pokémon other than Bulbasaur and want to make up for it.  Bulbasaur's hex number is 99, which converts to 153 in decimal.  Now, you could fight a wild Pokémon with a Special of 153, but the only place to find that is on a Level 64 Chansey in the Unknown Dungeon, and those Chansey are quite rare.  Worse, they only have a one-in-sixteen chance of having the right Special.  Fortunately, there is a solution: Ditto's Transform, which copies the stats of the opponent.

This means that you can catch any Pokémon in the game by following these steps:

  1. Catch/raise a Pokémon with the proper Special stat (153 for Bulbasaur).  You can find a complete list of which decimal value is needed for each Pokémon .
  2. Fly away from the Gambler battle.
  3. Fight any trainer that can see you from a distance and walk up to you.  (This is necessary to both unlock your Start button and trigger the Route 8 battle.)
  4. Find a wild Ditto and let it Transform into your Pokémon with the proper Special.
  5. Beat the Ditto (or just run away) and immediately fly to Lavender Town without fighting anything else.
  6. Enter Route 8, and Bulbasaur (or whatever) will appear.
Note that since only the least significant of the two Special bytes is used for this, you can also add 256 to the values listed above and use that Special to get the same Pokémon.  This is important because there are a few Pokémon, such as Kangaskhan, whose identifying value is lower than the minimum Special that any Pokémon can have.  In Kangaskhan's case, you can find her by using a Special of 258 (2 + 256).

Also note that when you perform this trick, any trainers you fight after the Gambler will be "used up."  Make sure you have enough unused trainers to catch all the Pokémon you want!  Fortunately, the Gambler can be used to initiate the trick an unlimited number of times.

by whitecat, mirrored from http://pokedex.kary.ca/catchmew.shtml

note: for more expansive coverage (without screenshots):
  mew glitch faq by thescythe at the gamefaqs pokemon red/blue page
  glitch guide by imakuni at the gamefaqs
pokemon yellow page