Scottish Natural Heritage has appealed to people in the Western Isles to report sightings of foxes.
Experts are unsure how foxes reached the islands
Foxes are not indigenous to the islands and the agency fears they could prey on birds and other wildlife.
SNH said foxes had been spotted in Lewis and Harris and asked people to get in touch with other sightings.
David MacLennan, area manager, said it was a mystery how the animal reached the Western Isles, which has problems with other non-native species.
He said: "There are two ways they could come - they either sneak on a boat and somehow or other get across without anybody noticing.
"The other option is that somebody took them here deliberately."
Hedgehogs are another non-native species and the policy of culling them on North Uist and Benbecula to protect ground-nesting birds has proved controversial.
The total number of hedgehogs reached about 4,000 since a handful were introduced in the 1970s.