Defog doesn't exist as a TM, nevertheless it does exist as a HM in Diamond, Pearl and Platinum. Getting the HM depends on the version of your game. In Pokemon Diamond and Pearl, you get the HM from an NPC when you enter the Great Marsh. You will need the Fen Badge from the Pastoria City's Gym in order to use it outside of combat. In Pokemon Platinum, you will find the HM deep inside Solaceon Ruins. You will need the Relic Badge from the Hearthome City's Gym to use it outside of combat.
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In pokemon diamond you have to go to the great marsh in pastoria city then talk to the ace trainer on the right right by the gate and you will receive it.
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there is no defog in pokemon black/white
in the great marsh
There is no way for you to get Defog in Pokémon SoulSilver, there is no Defog HM or TM in "SoulSilver" and there are no Pokémon that naturally learn Defog, the only way for you to get a Pokémon to know Defog in "Pokémon SoulSilver" is to trade the Pokémon over to "Diamond, Pearl or Platinum" and teach it Defog and then trade it back over to SoulSilver.
defog is not in emerald,
you get hm strength and hm defog after defeating two gym leader.Fantina(defog) Byron(strength).