Hunters working for the government's wildlife agency will be stalking ponds in south-west France this weekend, aimed with flash-lights, rifles, silencers and night-vision sights.

They have been mobilised for the most intensive effort so far to terminate a plague of giant Californian bullfrogs which is threatening to disrupt the ecology of the Gironde, Dordogne and several other d�partements.

The aggressive and voracious bullfrog (Rana catesbeiana), introduced illegally 37 years ago, can grow to more than 4lbs in weight and almost 2ft long. It consumes other frogs, fish, lizards and even small birds.

Since the frogs were first released, as a joke, on a private pond near Libourne in 1968, they have colonised ponds, lakes, marshes and gravel pits all over the d�partement of the Gironde. They have been found in the Landes area to the south and in the Dordogne, Lot-et-Garonne and Loir-et-Cher d�partements, further north.

Ecological groups have been warning for years that they must be eliminated to prevent the destruction of indigenous species. Had they been a delicacy for humans, the American interlopers might have been tolerated. Unfortunately for them, they are inedible (even their enormous legs).

Destroying the frogs is not easy, however. The Gironde fisheries protection association attacked a pond full of bullfrogs with electricity a few years ago. The frogs fought back. The hunters battled with them for two hours. They killed just one frog before they gave up.

Assaults on the frogs have also been made with nets and by draining ponds, to little effect.

Game-keepers and volunteers working for the Office National de la Chasse et de la Faune Sauvage (National Hunting and Wild-life Agency) have now developed night-fighting techniques. The frogs are easier to locate at night because their eyes reflect torchlight.

"Shooting them with rifles is the most effective method we have found," said an environmental campaigner, Luc Gueugneau.

"It seemed like a rather mean-spirited approach at first but we found that it was the best way of killing all the adults."

Even so, experimental attacks on ponds and lakes over the past 11 months have killed only 120 frogs. A much bigger offensive, starting this weekend, aims to exterminate all the bullfrogs in France within five to 10 years.