A 39-year-old man died after a taxi driver intervened in an argument he was having with his girlfriend.
Police said the man fell to the floor and sustained a serious head injury in the incident at about 0140 BST on Saturday at Sunnyside Road, Bridgend.
He was taken to hospital with serious injuries but has since died.
A 21-year-old male taxi driver has been arrested and bailed. Police are appealing for information about the incident.
Police said when the taxi driver arrived to pick up the couple, they were "involved in an altercation".
Officers believe he then attempted to "placate the situation".
"The incident has then developed, whereby the 39-year-old male partner has fallen to the floor and sustained a serious head injury," police said in a statement.
"He was conveyed to the Princess of Wales Hospital and was admitted with life-threatening injuries. Sadly, he has now died."
Ch Insp Stephen Jones said: "We would like to reassure the local community that this is an isolated incident that has ended tragically.
"There are also a number of rumours around that have not been substantiated such as a weapon being used.
"We can reassure the local community that this is not the case."
Police said a family liaison officer had been assigned to the victim's family.
Anyone with information about the incident is asked to contact the Bridgend major crime incident room on 01656 306103.