The girl was found stabbed to death in the bedroom of a house at Barnstaple, north Devon, yesterday morning. Neighbours discovered her 37-year-old mother on the doorstep bleeding from wounds. It was also suspected that she had taken a drugs overdose.
She is in the intensive care unit at North Devon District Hospital but she is conscious and her injuries are not life threatening.
A police spokesman said officers were waiting to question the woman at the hospital and were not seeking anybody else in connection with the killing.
Detective Chief Inspector Andy Pierce said: 'At about 9.30am, a call was received from a lady in the Roundswell area who had found her neighbour at the door bleeding from wounds.
'Police entered the house and found the body of the woman's nine-year-old daughter in an upstairs bedroom. It was later established she had died of stab wounds to the upper body.'
The girl's father is separated from her mother and no longer lives at the recently built semi-detached two-bedroom house. Police have traced him and broken the news. He was being comforted by relatives last night.
Police are trying to contact other relatives before releasing the name of the dead girl.