Bunbury Detectives are conducting inquiries into an incident on Saturday morning.
Around 2am, an altercation took place on Victoria Street involving eight to 10 people.
A 56-year-old man suffered serious head injuries during the ordeal.
The man was flown to Royal Perth Hospital, where he remains in a critical condition.
One male has since been arrested. Police are now looking to identify the other people involved.
Anyone with information has been urged asked to contact Crime Stoppers on 1800 333 000 or via crimestopperswa.com.au.