The Prison Service is carrying out an investigation after a disturbance at a young offenders institute.
It happened on Wednesday night at the Deerbolt institute near Barnard Castle in County Durham.
Police are helping to investigate the incident, which left one prisoner with burns after he was apparently scalded.
A prison spokesman said the 26 prisoners were contained and eventually dispersed. No demands were made and no other people were injured.
The disturbance began at around 1920 BST on Wednesday and was contained to one part of the prison.
A prisoner was treated at an outside accident and emergency department but was later returned to the institution, said the spokesman.
In February Deerbolt was the scene of a seven-hour siege resulting in tens of thousands of pounds of damage to the prison chapel.
The prison service inquiry will be looking at this and other recent incidents at the jail.