These codes once earned are enabled at the Weapons or game options screen
Do This To Get this
get gold medal in Basic Training mode. Worms bleed when shot
get gold medal in Super Sheep Racing training mode Aqua sheep
get gold medal in Euthanasia training mode Longbow power-up
get gold medal in Rifle Range training mode Shotgun power-up
get gold medal in Artillery Range training mode Grenade power-up
get gold medal in Crazy Crates training mode All weapon crates have sheep
get an Elite rank in Deathmatch mode Invincibility
Finish mission 4 Laser sight
Finish mission 8 Jetpack
Finish mission 13 Fast walk
Finish mission 16 Invisibility in net games
Finish mission 20 Low gravity
Finish mission 25 Indestructible landscape
Finish mission 33 Super Banana Bomb power-up
Gold medal and Elite rank in all Full Wormage game options

Tips for Deathmatch:
When you start a new Deathmatch, your opponents have all the skill level 1.
If you leave the game or change the team and continue it later with the first team, the opponents may get higher skills (that depends on your rank, e.g if you are "Highliy competent" you play against 4 teams, 3 of them with skill level 3 and one with skill level 4). But you can cheat the game by playing the Deathmatch COMPLETE FROM THE BEGINNING WITH ONE TEAM WITHOUT LEAVING THE MATCH. So you can fight against computer opponents with skill level 1 the whole time!
Info submitted by Psycho Wormage (KHWO8@t-online.de) on Tuesday, March 30, 1999 at 08:45:11

Get more Levels:
To Get a lot more cool levels first you have to beat basic training (it's really not that hard) and then you can gets tons of missions and when you beat a mission you get that level! some of the levels have cows, octopuses, buffalos, worms, logos, plants, wells, books and pumpkins in them and one level called "Horny Nuke" looks like it came out of a Dr. Seuss book! Also, in one level you have to parachute onto the enemy! And in this one level you have to fly a super sheep around a rocky cavern!