Staff at a London jail have sent letters to inmates thanking them for good behaviour, an inspection report said.
It added that well-behaved inmates at HMP Isis have also been issued with reward slips.
The category C training establishment sits within the perimeter of Belmarsh high-security jail in south-east London. It held 600 prisoners at the time of the inspection in July and August.
The report from HM Inspectorate of Prisons said:
While levels of violence at the prison had increased and were high, chief inspector of prisons Peter Clarke said the culture and atmosphere were "definitely improving".
HMP Isis is one of 10 prisons selected for a £10 million Government improvement programme.
Prisons Minister Rory Stewart, who has pledged to resign if the campaign fails to reduce violence and drug use at the selected jails, said: