Police are continuing their appeal for information into an armed robbery in Greenfields late in July.
A 23-year-old man was walked along Waldron Boulevard at around 10:45pm on 26 July when he noticed a man on a BMX bike heading towards him as he approached Wildair Street.
The rider of the BMX bike stopped and struck the victim to the back of the head, causing him to fall to the ground.
The offender threatened him with a knife before stealing his bag and riding off.
He was last seen riding east along Waldron Boulevard towards Murdoch Drive.
The victim was treated in hospital for a head and shoulder injury, received in the attack.
The offender is described as 18 to 20-years-old, with brown skin and a slim build, approximately 178cm and with short dark hair.
He was wearing shorts and a hoodie.
Mandurah Detectives would like to speak to anyone who was in the area at the time of the incident who may have seen a person or a BMX bike.
Anyone with any information is urged to contact Crime Stoppers on 1800 333 000 or make a report online at www.crimestoppers.com.au.