An old Uzbeki pensioner wearing a turban slowly climbs the 36 holy steps of the ancient Shakhi-Zinda Moslem cemetery and threads his way past tourists to the mausoleum of Kussam Ibn-Addas.

As camera-toting Russians and East Europeans gawk, the old man kneels to pray before the gilded, multitiered tomb of the holy man who tradition says brought Islam to this fabled city 13 centuries ago.

The Uzbecki and his huge and growing tribe are part of an ancient empire of Moslem-oriented custom and belief that endures today in the heart of the Soviet Union's Central Asian republics of Kirghizia, Tadzhikistan, Turkmenia and Uzbekistan.

The Soviet Union is, in fact, the world's fifth most populous Moslem nation, behind Indonesia, Pakistan, Bangladesh and India. And the Soviet Asian population is growing far more rapidly than the long-dominant European Russian citizenry west of the Urals.

Russia's rulers will face complex and important questions in the years ahead when Soviet Asians become the single largest source of new manpower for the armed forces and industry.

Less than one-third of the people of this region, for example, speak Russian-and that minority may be decreasing.

The challenges this will pose are accentuated by the fact that after more than 50 years of rule from Moscow, that capital when viewed from this region seems remote and almost foreign.

The Soviet citizens here feel a deep ethnic bond to the peoples of Iran and the Middle East.

These ties help explain, along with geopolitical considerations, the Kremlin's keen interest in the current turmoil in neighboring Moslem Iran, and Moscow's quick moves to solidify a strong position of influence with the struggling new Marxist regime in neighboring Moslem Afghanistan.

Everywhere in this colorful, polyglot region are the threads of ancient Asian-and Moslems-heritage.

Although Slavs who have come from west of the Urals to guide and share in growth of the booming region clearly control the Soviet-oriented levers of economy and society, the pace of life and many of its goals are stamped by the outlook of the Asians.

As perhaps nowhere else in the Soviet Union, Moslem-oriented traditions and folkways of central Asia seem to have blunted the sharp, atheistic thrust of official Soviet industrial culture.

The spires and domes of blue-tiled mosques, mausoleums, and minarets mark the skylines of such fabled cities as Samarkand, Bukhara and Khiva, drawing thousands of foreign tourists to look upon the splendors-and excesses-of the once-mighty Islamic nations of the region and ponder their meaning for today.

The changes wrought in the region by Soviet power are enormous: the 98 percent illiteracy that existed in 1918 has been wiped out. Clean water supplies, adequate food and medicine and other modern public health standards, bought at enormous cost to the government, have almost doubled life expectancy.

Education, once confined to a tiny, privileged minority and tightly controlled by Moslem mufti, has been freely available since early in the Soviet years. About 20 percent of Central Asia's young people go on to some kind of higher learning institutions after high school, according to statistics.

Like the American "Sun Belt," Soviet Central Asia is a boom region of agriculture, heavy industry, and gas field.

It boasts the world's longest irrigation canal, bringing water more than 600 miles through the Kara-Kum desert in Turkmenia to create new farming regions resembling in potential California's San Fernando and San Juaquin Valleys.

Five million of the 8 million tons of cotton harvested this fall in the Soviet Union came from Uzbekistan, and the fruits, nuts and vegetables of Thereon are prized throughout the produce-short country. In the past 30 years, savage earthquakes have leveled several of the largest cities, including Tashkent. Yet phoenix-like, they have risen more powerful and important from the rubble, drawing strength from across the Soviet Union.

Above all, the Central Asians in the coming decades will be Russia's single most fertile source of manpower. The birthrate here is more than double that of European Russia, where the dominant Slavic population is virtually stable.

By the end of the century, some Western demographers estimate, about one-third of all 18-year-old military conscripts in the Soviet Union will be of Central Asian origin, speaking Turkic or Persian-based native languages and struggling with Russian-language instruction manuals.

The implications of this are enormous.

The Central Asians share a history markedly different from the Slavs,-one tied to the Persian and Arab past.

The region's noble past, personified by Tamerlane the Great, whose empire brought Central Asia in the 14th century to a peak of intellectual and commercial sophistication unmatched since, survives today in monuments and local histories.

But the region is firmly webbed by television, radio, and cheap air and railfares into the outer world and the affairs of the powerful Soviet state.

This modernity and the levelling influence of authoritarian industralization has meant a healthier, materially better life for Soviet Central Asians than for their ethnic cousins over the border in neighboring Iran and Afghanistan.

This circumstance of well-being, which Marxist historian Roy Medvedev in a recent interview suggested may ultimately lead to "bourgeois" attitudes of conservative self-content throughout the nation, has meant wide acceptance of the Soviet way of doing things among peoples who have little in their pasts that would ally them with the Soviet power.

Indeed, in contrast to the Baltic republies of Estonia, Latvia, and Lithuania or the restless Trans-caucasian republics of Georgia and Armenia, strong nationalist and anti-Soviet attitudes have not emerged here in the form of either covert spokesmen or flourishing samizdat -underground, self-published literature.

Yet, as surely, as material well-being binds the Soviet Asians into the Soviet Union, custom, history, language, religion and blood-those powerful shpers of human events - bind Soviet Asians into the Moslem world as well.

In random interviews during a recent two-week trip though Uzbekistan and Turkmenia, it was impossible to find a Soviet Asian who did not first describe himself as a "Moslem" an dthen specify nationality.

Many of these people said that while they no longer practiced their religion, they had Korans at home and tried to read from them and learn the lesson of Allah.

Many were not to be found at Friday "Namaz," or prayer, when muezzins call the faithful and small, officially sanctioned congregations of Soviet Moslems turn their gaze and thoughts toward Mecca like tens of millions of their brethren from Cairo to Jakarta. Others scoffed at the idea of praying five times a day as required by Islam. "There isn't time enough in the day," said one young Turkmeni intellectual in Ashkhabad.

Western experts have estimated the number of Soviet Moslems at anywhere from 30 to 50 million, based in part on the assumption that to be an Uzbek, Tadzhik, Tartar or Kazakh implies belief in Islam.

Indeed, it is impossible to say how many Central Asians are "believers," in the religious sense on the word, or simply call themselves Moslems as a generic term denoting the marked difference between themselves and the Slavs. The Soviets do not keep count, on grounds that "religion is free" in the Soviet Union. But with a rigid framework of service to the Soviet state, religious Islam can be said to be flourishing in a small way. The days of brutal Stalinist repressions of the 1920s are long gone.

World War II changed things for the Central Asians, who fought against the Nazis and joined in the war effort at home. Asian faces are memorialized here in the same kind of heavy war monuments found in profusion in European Russia, where 20 million were lost.

Stalinist suspicions of collaborationist impulses faded. In 1946, the government allowed the reopening of one small Medressah, or Moslem holy school in Bukhara, which still functions today. In the years since the government, in the same heavyhanded way that characterizes its approach to the Russian Orthodox Church, has pursued a double strategy - balancing cunning suppression with a carefully mapped, slow, but steady expansion of approved Moslem religious practice.

In the postwar years, Moslem religious practice advanced under the watchful eye of the Soviets so that there are now more than 140 functioning cathedral mosques and more than 1,000 smaller parish mosques in the four republics. New mosques are being opened almost every year. Millions of rubles are being spent to rebuild and refurbish the fabulous Moslem architectural monuments of the past.

The Soviets, proudly pointing to all this as refutation of Western criticism the religion is suppressed here, eagerly publicized their own efforts at preserving and enhancing the status of Islam in Central Asia.

Like so many similar Soviets claims, it conveys many meanings and achieves many purposes, not all of them easily visible. CAPTION: Picture 1, Soviet Moslems pose before going to Friday prayer services near Samarkand.; Picture 2, An Uzbekistan family heads home after shopping at the bazaar in the old quarter section of Bukhara. Photos by Kevin Klose-The Washington Post