Pokemon Crystal: Hacking Wild Pokemon Encounters
Version 0.4
By Tzepish Tachman (Blaine Higdon), tzepish@scientist.com

Click here to go straight to the data tables.

Wanna' screw the game up? Here's how.

By the way, I'm gonna' assume you know hexadecimal and how to use a hex editor. If you don't, then, rats.

Each area has 21 possible wild pokemon encounters, 7 for each time period (Morning, Day, Night). The data for each encounter is stored in two bytes: the wild pokemon's level, the wild pokemon's number (pokedex number, converted to hex).
An example:

0B9B = Level 11 Cyndaquil
(11 = 0Bh, Cyndaquil's pokedex number is 155 = 9Bh)

Anyway, at each area, you'll find 21 entries like that above, each representing one of that area's encounters. The areas are separated from each other by some more hex... Let's look at an example. The pokemon data for Route 29, the game's first area, begins at 2AE02h.

0210 02A1 0310 03A1 0213 03BB 03BB 0210 02A1 0310 03A1 0213 03BB 03BB 02A3 0213 03A3 0313 0213 03A3 03A3 1A01 1919 19

The first 7 are the morning encounters, the second 7 are the day encounters, and the last are night encounters.

Let's interpret:

Morning: L2 Pidgey L2 Sentret L3 Pidgey L3 Sentret L2 Rattata L3 Hoppip L3 Hoppip

Day: L2 Pidgey L2 Sentret L3 Pidgey L3 Sentret L2 Rattata L3 Hoppip L3 Hoppip

Night: L2 Hoot Hoot L2 Rattata L3 Hoot Hoot L3 Rattata L2 Rattata L3 Hoot Hoot L3 Hoot Hoot

The 1A01 1919 19 entry acts to seperate Route 29's data from Route 30's. Immediately after that last 19 at the end of the Route 29 data, you'll find the first entry for Route 30, 03A5, L3 Ledyba. Each area is seperated by 1919 19 (sometimes 0505 05). The 1A01 preceding it is different for each area (it's 1A02 at the end of Route 30's data), but it's always there. I don't know what the differences mean, so it's probably wise to leave them alone.

So far, it seems you only have to add 2Fh to each address to get the next area's address, though I'll probably need to investigate this further. For instance, where is the encounter data for Dark Cave? Will it come before or after some of the Routes? I'll have to sit down and mess with the data for a bit.

Expect updates. In future versions, I will add to the data tables below. I'd also like to look into the data for the starting 3 pokemon choices. 39AF2h looks interesting.

Email me for corrections, comments, etc.

Here're all the data, and the meaning behind them: