Four people have been arrested at Parc Prison over allegations of assault and misconduct.
Detectives they're investigating following reports of a series of incidents - and concerns about staff conduct - at the Bridgend jail on Wedneday.
Three men, aged 25, 35 and 45, from the Llanelli, Merthyr and Pontcymmer areas are being questioned, along with a 23 year-old woman from Bridgend.
South Wales Police has stressed its enquiries are at an early stage.
Detective Chief Inspector Dean Taylor said: "We received concerns about the conduct of staff at HMP Parc on Wednesday September 18."
"Officers are at the early stages of the investigation and are working closely with G4S while inquiries are ongoing."
Parc Prison has been at the centre of controversy in recent months.
13 inmates have died so far this year with at least four of the deaths being linked to drugs.
In June, its director Heather Whitehead resigned after just ten months in charge and was replaced by Will Styles.
But earlier this week, the social justice minister, Jane Hutt, insisted there had been "considerable progress" in the day-to-day running of the jail, describing the situation as "much, much more stable".

Vueling withdraws winter flights to Aicante this winter
Playground in Barry forced to close after fire damages equipment
Campaigners Take Fight for Fairer Pathways in Welsh Women’s Football to Parliament
Vale Councillor Gwyn John Launches His First Novel at Llantwit Major Book Signing
Pink poppy display commences with celebration of life
Former Barry office block converted into new homes for people at risk of homelessness
Welsh Government publishes independent report on Level 4 intervention at Cardiff and Vale UHB
Three CoastSnap stations set to be installed along the Glamorgan Heritage Coast
Daffodil Runner statue installed at Marie Curie Hospice in Penarth
Woman Dies After Incident in Leckwith as Police Investigation Continues
Vale entrepreneur wins national Innovate UK award for sustainable construction technology
Cardiff Airport retains top accessibility rating as regulator reports record demand for assistance
Open Doors event to showcase hidden world of Glamorgan Archives
New details have emerged of a new tree-lined public square that will replace a traffic dominated junction
Dyffryn Gardens’ Edwardian rose garden blooms again after decade-long restoration
Barry’s Own Daniel White to Walk 190 Miles Back to Where His Round Table Story Began
1,300 people attend Flying Start Family Fun Day
Dispersal order put in place to tackle anti-social behaviour
Interim Chief Executive Appointed at Cardiff and Vale University Health Board
Cardiff Theatre Marks 50 Years of Service for Long‑Standing Staff Member