The men weren't handcuffed when the guard put them in a cell during a break in the trial this week, and an investigation will be conducted on how it happened, Allen County Sheriff Sam Crish told The Lima News.
He said authorities broke up the altercation within two minutes, and the men suffered only minor injuries.
The holding cell was out of public view, but the fight was recorded on video that the judge allowed the jury in the suspect's trial to see Thursday after denying a defense request for a mistrial because of the fight.
WLIO-TV says the video shows defendant Markelus Carter punch the other inmate, who is then seen putting Carter in a headlock and throwing some punches before Carter bites him on the arm and guards break up the fight.
Court records show Carter, 46, is on trial on aggravated murder and weapons charges. He has pleaded not guilty in the 2009 slaying of Kenneth Warrington. Prosecutors say Carter was upset that Warrington was having a relationship with his ex-girlfriend.
A message seeking comment from Carter's attorney, Jon Paul Rion, was not immediately returned Friday.