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How to Catch Mew in Pokemon Red and Blue: The Glitch, and Why It Works

Mew was never in the game to be caught. It was handed out at events, behind glass. And yet, with no device and no cheat, a level 7 Mew walks into a wild battle. The reason is a single number the game forgot to clean up.

By Christopher Robert Fox | Decoded from a Pokemon Red ROM, verified against the disassembly

Every wild encounter in Pokemon Red and Blue starts by reading one byte: which species to send out. Normally the game sets that byte on purpose. The Mew glitch is what happens when it does not, when an old value is still sitting in that spot from something you did minutes ago. That leftover value is 21, and species number 21 is Mew. The animation above is that exact moment: a number becomes a Pokemon.

The one number

Mew's internal index in Red and Blue is 21. Not its Pokedex number, 151, but the order it sits in the game's own species table. Leave a 21 where the wild species is read, and the game spawns Mew, at level 7, ready to catch.

The one number

The game keeps a list of every species in a fixed order, and it fetches the wild Pokemon by its place in that list. Not by name, by number. Turn the dial and see what each leftover value would summon. Most land on ordinary Pokemon. A few land on nothing, and the game shows you MissingNo. One of them, 21, is Mew.

Every value 0 to 255, read as a species by its place in the game's internal order (verified from the ROM). Gaps in that order, and everything past its end, come back as MissingNo.

What Trainer-Fly actually does

So where does the leftover 21 come from? From a trainer battle you start but never finish. Walk through it.

Why a battle you flew away from comes back as a wild Mew. The exact route and setup are the glitch community's work; the anchor, that species 21 is Mew, is read straight from the ROM.

The steps

Here is the foolproof route, the one most people use. Tick through it. It works on a real cartridge, no tools, no cheats.

    A wild Mew
    The standard method, worked out and refined by the Pokemon glitch community over the years. Trainer numbers and small details vary between Red, Blue, and Yellow.

    Meet Mew

    The Pokemon at the end of that number. Here it is, straight from the cartridge: its sprite, the base stats the game gives it, and its cry, rebuilt live from three bytes of sound data. Not a recording.

    Mew, decoded from the ROM
    MEW No. 151
    PSYCHIC
    Mew's real base stats and cry, read from a Pokemon Red ROM. Every stat is 100, the only fully balanced Pokemon in the game.

    Why level 7

    The level is not fixed either. It comes from another leftover number: a stat stage. Stages run from minus six to plus six, and the game stores them as one through thirteen, with zero stored as seven. An untouched setup leaves a seven in that slot, so you get a level 7 Mew. Nudge the stat first and the level moves with it.

    Stat stage to level: the game stores a stage as (stage + 7), and the Mew glitch reads that stored value as the level. The one-to-thirteen storage is the game's own; that it becomes the level is the documented glitch behavior.

    The same door, other numbers

    21 was kind to you: a real Pokemon, catchable, alive. The exact same trick with a different leftover number opens the door on something stranger. The read does not care what it lands on. Pick a number.

    The wild-species read never checks whether the number lands on a real Pokemon or a gap in the list. Feed it 21 and you meet Mew; feed it a gap and you meet MissingNo.

    Does it still work

    This is a thirty-year-old bug in a cartridge, so a fair question is whether it still runs on anything you can actually hold. It does, on more than you would guess, and the Mew you make this way is real enough to carry all the way to the newest games.

    The exploit is Generation 1 only, but the same ROM ships on the cartridge, the 3DS re-release, and every emulator, so the setup runs on all of them. A Mew caught this way is a legitimate, tradeable Pokemon.

    From the ROM

    None of this is our claim to make. The number is written down in the game's own source. Here is the real list of Pokemon IDs from constants/pokemon_constants.asm. Count down from zero, or just tap a line: Mew sits at $15, which is 21.

    The exact constants file the game is built from. The const macro just counts up from zero, so MEW lands on hex 15, decimal 21, the number the glitched read pulls out of memory.

    And the letters are just as real. Every glyph on this page is read out of the cartridge font, eight bytes each, one bit per pixel. Watch M, E, W decode the way the Game Boy draws them.

    Eight bytes from the cartridge. One bit per pixel.
    The real font bytes from the ROM decoding into the letter in front of you. One bit set is one pixel of ink, on a white screen, exactly how the game does it.

    Try it with your own name

    The same read that turned 21 into Mew is the engine behind another famous glitch: the old man on Cinnabar's coast. After he shows you how to catch a Pokemon, the game briefly hands the wild-encounter routine your NAME and reads its letters as species. Type a name and watch each letter become a Pokemon, or a gap.

    Each letter is one byte; the byte is read as a species index. Letters that land on a real number give a real Pokemon; letters that land on a gap in the list give MissingNo. The full Name Glitch calculator works out the exact encounters, and levels, the Cinnabar old man produces from a name.

    Where to go next

    You now know the number, the read, and why it lands where it does. Two places to take it further.

    Battle engineFight a Lv 7 MewThe Gen 1 battle system, rebuilt from the ROM. Send out the exact Mew this glitch catches and take a turn. Name Glitch toolYour name, decodedType any name and see the full old man glitch encounter table it summons, cries and all, straight from the ROM. The GlitchhouseEvery specimenMissingNo, 'M, Mewtwo and the rest of the family this same door opens onto.

    A live decode, Mew's own cry, and more are still on the way. The route is decades of shared work by the Pokemon glitch community; what we add is the view from the ROM.

    Go deeper: the Gen 1 battle engine | browse the Glitchhouse | arbitrary code execution.