Health officials are investigating at another school closed on Monday because of E.coli.
The first case of E.coli at the nursery was notified on Wednesday
It has been confirmed that a six-year-old girl at St David's Church in Wales school in the village of Colwinston has contracted the bug.
It comes after a Brecon nursery was shut over the weekend when a number of cases were confirmed there.
The outbreak has affected nearly 170 people and led to the death of five-year-old Mason Jones.
Over 40 schools across the valleys and surrounding areas have been linked to the outbreak since September.
Mason Jones, five, died after contracting the E.coli infection
The St David's Church in Wales pupil, is currently being treated for the infection at the Princess of Wales Hospital in Bridgend.
Vale of Glamorgan Council environmental health officers have launched an investigation at the 164-pupil school in conjunction with the National Public Health Service for Wales.
"The school decided to close as a precautionary measure, and a decision on reopening will be taken in the light of investigations conducted by our environmental health officers," said a spokesman.
Meanwhile, health officials are investigating the infection of a private nursery the Brecon. They have said it is not known if the infection is connected with the south Wales valleys outbreak.
The first case at the Puffins pre-school nursery, which is attended by 54 children, was identified on Wednesday with more coming to light over the weekend.
Public health officials took the decision to close the Brecon nursery over the weekend while tests were carried out.
An intensive clean will take place on Monday at the premises.
Parents of the pupils, who range from five to 10 months, have been contacted by environmental health officers.
A private meeting for parents will take place at 1800 GMT on Monday at the Powys Council offices at Neuadd Brecheiniog in Brecon.
Families will be offered one-to-one advice and health officials will arrange for samples to be taken from the children.
Dr Marion Lyons from the Public Health Laboratory said: "Obviously if there is any risk in the nursery we will identify it now.
"There will be in-depth cleaning so parents will be reassured when they come back to the nursery it is a clean and safe environment.
"We would encourage all parents in the community to be vigilant... Hand washing and personal hygiene is very important," she added.
While it is not known whether these cases are linked with the outbreak in south Wales, health officials say it is likely that E.coli 0157 remains in the community due to the widespread nature of its original distribution.
The outbreak control team urged parents to keep vigilant and let their doctor know if their child has bloody diarrhoea.
The Welsh Assembly Government, which has promised a "no-holds-barred inquiry" into the outbreak, last week appointed food safety expert Professor Hugh Pennington as chairman.
Professor Pennington carried out an inquiry into an outbreak in Scotland in 1996 which killed 17 people.