Detectives say there's been a rise in heating oil thefts across Anglesey and Gwynedd since the start of the year.
North Wales Police say they're warning residents and business owners to be vigilant for suspicious activity.
Reports of thefts have been receieved from the Amlwch, Caernarfon, Criccieth, Pwllheli and Rhosenigr areas in the last three months.
Officers are advising owners of oil tanks to keep them secure by fitting them with a padlock or lockable filler cap, installing lockable gaps or securing the tank within a cage.
They also suggest hiding the location of the tank and fitting security lights and CCTV cameras or an electronic gauge, which can activate an alarm when the oil level drops.
Anyone with information about oil thefts should contact North Wales Police on 101 or Crimestoppers anonymously on 0800 555 111.
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