It was at 23.30 last Tuesday evening that the police were called to an address in Fagersta after a fight in an apartment.
A dead man was found at the scene and in the apartment there was another man whom the police arrested on suspicion of the crime.
The man is now also under arrest, suspected of the crime.
A preliminary investigation regarding murder or manslaughter has been initiated.
According to the police, the men were acquaintances and at the moment there are no more suspects.
An interrogation will be held on Wednesday and a technical investigation will be carried out.