The curious incident of the fox in the night-time

Captured in the Tudor and Georgian rooms of the National Portrait Gallery in London, this fox is no optical illusion - and neither is he an accidental tourist.

As part of an investigation into the omnipresence of surveillance cameras in London, the fox was introduced into the gallery by Francis Als, a Belgian artist who lives in Mexico. For several hours one night early last year, Als used the gallery's security cameras to record the animal pondering a few paintings and, more frequently, checking out pieces of chewing gum stuck beneath the benches.

The footage, commissioned by the contemporary art group Artangel, has been edited into a 20-minute film, called The Nightwatch, which is now on display in the main gallery of the National Portrait Gallery.

"The fox is a very lazy animal and what he does is range the territory for whatever time it takes him to go through the rooms. Then he just finds a place to nest and he starts sleeping," Als said.

"His active time is quite short. There's no food and no kind of danger. He gets very quickly bored because it wasn't very interesting hunting territory." Als would not reveal the origins of the fox, named Bandit.

The idea was very simple but took some time to organise as other institutions had refused to take part for fear of compromising their security arrangements. Earlier proposals to attempt something similar amid the CCTV cameras of the City had also floundered.

However, the National Portrait Gallery agreed because its cameras are already a very visible deterrent to its visitors - in contrast with rivals who hide theirs away.

"I hope that my pieces are very open to different readings," Als said. "But what I wanted to do was to address the surveillance systems in London, the omnipresence of cameras all over the city."

Sandy Nairne, the gallery's director, said he hoped showing the film might attract new audiences to the National Portrait Gallery who might also decide to explore other Artangel commissions.

He had no qualms about letting the animal in. "He was a very well-mannered fox called Bandit. And we were extremely careful that there is not a jot of information given away about security in this video that you can't see in the galleries already," he said.

"We have excluded anything that could compromise security. It might remind people in a way they're not normally reminded that they are on camera in institutions such as ours, though that wasn't the intention."

The fox project was just one of several carried out by Francis Als for Artangel. He recently described them as "a discursive reading of one location, being London".

Other films, videos and paintings in the sequence include Railings, which, he says "explores the rhythmic possibilities afforded by a characteristic feature of Regency London, its railings," and Guards, which follows 64 individual Coldstream Guards as they travel in the City.

Als has spent the past five years walking the streets of London to examine the rituals and habits of its citizens and to conceive his work. With the exception of The Nightwatch, all the resulting work is on display at 21 Portman Square, London, W1, until 20 November.