Samples are to be taken of oil that washed up on Perth beaches on Saturday to determine if it was spilt from a ship or occurred naturally.
The Department of Transport says oil was found at Port and Leighton beaches to the south and as far north as Mullaloo and Sorrento beaches.
The ABC reports the oil could have been spilt from a ship or come up naturally from below the sea bed and been swept ashore by strong winds.
Fremantle Mayor Peter Tagliaferri says authorities will decide on Sunday whether to close the beaches.
