Neath Port Talbot Council is warning there is a high risk of landslip.
People living on Cyfyng Road have been asked to evacuate their properties by Neath Port Talbot Council.
A Neath Port Talbot spokesperson: "The decision to ask people to leave their homes at such short notice has not been taken lightly, but this is a matter of public safety.
"Following monitoring and investigation of the landslips at the rear of 81 to 96 Cyfyng Road, we are of the opinion that there is a high risk of further collapse and consequently a risk to the occupiers if they remain in these properties.
"Based on recent inspection findings and expert advice, we are now satisfied that Category 1 hazards exist at the properties which pose an imminent risk to the health and safety of occupiers and visitors and, as such, immediate emergency action had to be taken.
"Council officers from relevant services have been in Ystalyfera to offer advice and support to the residents who are affected.
"The assistance is on-going and it is the Council's intention to secure alternative accommodation for the residents affected."