"It's the hardest thing I've ever done," gasped Travis Tucker, a U.S. Marine veteran of Vietnam. "Much worse than I expected."

Tucker, 27, formerly of Arlington, Va., was describing the 100-mile march to the from the top of the Zambezi Valley escarpment, part of the training of volunteers in one of Rhodesia's elite jungle-fighting units, the Selous Scouts.

The heat made every mile an ordeal. There were 50 pounds of rocks in each trainee's knapsack and a steady stream of abuse from the instructors.

Those who survive the march and other endurance tests during training win the right to become Selous Scouts.

Tucker, who is one of three Americans in the organization, served with the Marines two years in Vietnam. He has lived in Rhodesia five years and when he got his call-up papers (two-year residents are liable for service), he elected to join the scouts.

"The Marines were the best at storming beaches and that sort of conventional warfare, but these guys are the best when it comes to the bush," he said. "If you sent 10 Marines into the Zambezi Valley against 10 Selous Scouts, you'd end up with 10 dead Marines."

The scout unit, named after 19th century hunter Frederic Courteney Selous, was created in 1974 when the army decided it needed a corps of bush trackers to fight a growing number of guerrilla infiltrators.

The unit is considered the most exacting in the armed forces, and the most controversial, too.

Until recently the Scouts operated under a veil of secrecy.Maj. Ron Reid-Daly, founder and commander, says this was intended to protest the unit's black members, who outnumber their white comrades four to one.

Since December, 11 Roman Catholic missionaries have been killed in terror attacks, and black nationalist leaders say black Selous Scouts masquerading as insurgents committed the crimes to discredit the guerrilla movement. The blacks are "The most valuable" soldiers he has says the major. If a black scout home on leave is identified to guerrillas as a Selous Scout, "He's had it, he's finished."

The mystery surrounding the Scouts inspired legends of gore and bravado, which gave way to accusations of foul play.

"We have rolled back the curtain because we don't like these allegations," said Reid-Daly.

Repors were allowed to spend four days with the scouts recently, for an unprecedented glimpse into the workings of a highly trained, highly motivated unit whose chief task is tracking.

Tucker, who is divorced and has a 5-year-old son living with his grandfather in the United States, was among 20 men left of the 80 volunteers who began the Selou Scout selection course two weeks earlier.

During the course the men can quite and leave Camp Wafa Wafa, which in the Shona tribal language means "you're dead, you're dead."

The emphasis during training is on bush survival. The instructors tell the recruits they can die of thirst or hunger while in the bush only through ingnorance. Before a march recruits are fed rubbery baboon flesh and meat so rotten it is green.

"There is absolutely nothing wrong with rotten meat, even if it's crawling with maggots, providing you cook it first and eat it while it's hot," says Sgt. Maj. Anthony White.

The idea is that if a trooper runs across a lion kill, he can make a meal of the remains.But the bush also provides delicacies. With a few swift cuts, White butchered a recently killed kudu, a buck. The recruits feasted on it and then were shown how to turn the stomack into a water container and make string from the sinew.

Besides bushcraft, recruits receive lessons intracking, skydiving, snorkeling, horseback riding and demolition to produce what Reid-Daly believes is "probably the finest counter-insurgency unit in the world."

Reid-Daly claims his men have been responsible for killing more than 1,200 of the approximately 2,600 guerrillas slain in the last four years. He said the scouts have lost about 10 men; five of them in one ambush.