Amid the tall grass and thick scrub of the remote, northern Zambezi River Valley, one of Africa's oldest and rarest creatures is making a past stand against a deadly human enemy.

The black rhinoceros, pushed to the brink of extinction in most of the continent over the last two decades, still thrives in this rugged terrain about 250 miles north of Harare. There are about 1,500 black rhinos in a 4,000-square-mile area. Outside of Tanzania, Zimbabwe is believed to have the largest concentration of the animals in the world.

But poachers, lured by top prices and an international demand for rhino horn, recently have invaded the valley. In January, wildlife officials found rhino carcasses there with their horns hacked off -- the first instance of organized rhino poaching in memory in the government-protected wilderness area. Since then, the official figure has risen to 23, and it is possible that dozens of others lay undiscovered in the rugged bush.

The poachers, armed with high-powered hunting rifles and operating in well-coordinated gangs, slip over the river in long canoes from neighboring Zambia. They return with horns that begin a winding, clandestine trail through the capital in Lusaka, on to Zaire, Malawi, Burundi and Sudan, and eventually to North Yemen and the Far East.

The government here has reacted by stepping up park patrols and even sending troops into the area. Soldiers have told visitors that there have been several exchanges of gunfire and that Zimbabwean motorized patrol boats have capsized Zambian canoes crossing the river illegally, plunging their occupants into the crocodile-infested waters.

Earlier this week the government confirmed that one accused poacher, a Zambian national, had been killed. At least eight others have been arrested and await trial.

But while conservationists praise the official effort, they believe more needs to be done and quickly.

"There's no doubt the black rhino faces total extinction," said Richard Pitman, chairman of the Zambezi Society, a Harare-based conservation group.

Recalling how the white rhino population of East Africa fell to a few dozen before steps were taken to save it, Pitman said, "We've got one of the few remaining populations of black rhino, and it's vital we find a way to protect it now and not wait until we reach a crisis."

The rhinoceros may be the world's oldest mammal, having existed in virtually the same form for 20 million years. It is uncannily well-suited for survival, a vegetarian that weighs up to 3,000 pounds but can gallop up to 35 miles an hour. Even lions defer to the rhino.

Two decades ago, there were as many as 200,000 black rhinoceroses in Africa. Now, according to the Kenya-based International Union for Conservation of Nature, there are perhaps 9,000.

The reason is its foot-long, 10-pound horn, a compacted mass of matted hair and gelatin. In North Yemen, daggers with jewel-encrusted rhino-horn handles have become a royal status symbol for young men, who may pay as much as several thousand dollars for one. In Taiwan, China, Korea and Hong Kong, the rhino horn powder can sell for up to $4,000 per pound and is prized as a traditional medicine even though various medical studies have demonstrated, as Pitman puts it, that "you'd do just as well chewing your own fingernails."

The hunters sell the horns for prices ranging from $20 to $100 to businessmen who smuggle them via international airports and harbors, where export procedures are lax and contraband easily concealed using measures such as false-bottomed trunks, forged export licenses and bribed customs officials.

In satisfying the black market demand, poachers have totally eradicated the rhino population of Uganda and Sudan and reduced the population in the Central African Republic to about 150 from a 1980 total of 3,000. In Kenya, where the black rhino population has fallen to about 550, the government recently announced a $2 million plan to relocate most of the survivors in sanctuaries surrounded by solar-powered electric fences designed to keep them in and poachers out.

Having dried up the East Africa supply, the poachers gradually have turned south. First they moved into Zambia's Luangwa Valley Game Reserve, where the rhino population over the last decade has fallen from about 4,000 to perhaps 1,000. Antipoaching patrols, privately financed by the World Wildlife Fund, have arrested more than 1,000 illegal hunters in the past five years.

Now they have turned to Zimbabwe. Graham Child, director of Zimbabwe's National Parks and Wildlife Management, estimates the country may have lost a million dollars' worth of rhino horn so far this year. But because Zambian nationals are involved, government officials here have refused further comment.

Sources here say those captured all have been Zambian villagers who clearly are being funded and equipped by outside persons. One of the rifles recovered from accused poachers has been traced to Zambia's national parks department in Lusaka, leading to speculation that the poachers are connected with officials there.

In response to the problem, the Zimbabwean government is planning to budget for an extra 550 game scouts nationwide, and to relocate 50 rhinos annually to the better protected Hwange game park in western Zimbabwe.

The Zambezi Society has launched an international appeal for funds to supply the antipoaching units with four-wheel-drive vehicles, two-way radios, weapons, camping gear and boats. But society head Pitman acknowledges these are only short-term measures.

Two years ago, North Yemen banned importation of the horns, but conservationists estimate the country still accounts for nearly 50 percent of demand. It will take stronger measures, they say, including a coordinated crackdown in the African capitals and ports from which the contraband is smuggled, to strangle the international market for rhino horn.