MOSCOW - In the minds of millions, the environmental disaster that is the Soviet Union is summed up in one word: Chernobyl.

The world's worst nuclear disaster ever, the 1986 power-plant explosion that spread radiation as far as the United States, has taken at least 250 lives, forced the evacuation of 115,000 people and cost $12.8 billion in cleanup and lost energy. The catastrophe continues to unfold in leukemia cases now being diagnosed.

But horrific as it was, Chernobyl was in some ways an environmental sideshow for the U.S.S.R.

The country today is facing not just one environmental crisis, but a series of overlapping threats that have developed over 50 years. Its outdated and inefficient heavy industries are environmentally as well as financially costly. An estimated 50 million Soviet citizens are living in cities and towns beset by serious air pollution.

The environment was the first cause taken up by the public after the perestroika restructuring, and has since become a metaphor for the malaise at the heart of the Soviet system: the destructive mindlessness of central planning, the seemingly impregnable power of a huge bureaucracy.

For many of the U.S.S.R.'s non-Russian republics, the environmental crisis warns of an even greater threat: the danger of national extinction posed by heavily polluting and dangerous industries arbitrarily imposed by Moscow. From the restless Baltic states in the northwest to Armenia in the south, the Soviet Union's nationalities are less and less willing to see their homelands bear the environmental brunt of national industrial development.

Mikhail Gorbachev's rule has seen a surge in environmental awareness. Environmentalists are given a sympathetic hearing in the official media. Grass-roots organizations have sprung up all over the country. Radical environmentalists have been elected to the Supreme Soviet, the nation's parliament. Anti-nuclear lobbyists have direct access to Gorbachev. And a biologist who is not a party member was given ministerial rank to oversee the new State Committee for the Preservation of Nature.

But throughout the country, the transition from awareness to actual improvement has been slow.

The U.S.S.R. spends about 1.5 percent of its gross national product on its battered environment, the new nature minister, Nikolai Vorontsov, said, adding that anything less than 3 percent is first aid. Vorontsov would like to see that figure rise to 5 percent.

Central Asia is being poisoned by the pesticides used to help its major crop, cotton, grow. Soviet statistics are notoriously unreliable, but independent specialists believe pesticides and other chemicals at levels up to 20 times the national average are used on farmland in the republics of Uzbekistan, Azerbaijan and Turkmenistan. Until recently, the pesticides reportedly included DDT.

The cotton republics report dramatically high incidences of anemia, hepatitis, miscarriages and birth defects. And Vasily Selyunin, an economics columnist who researched the situation there late last year, quotes local farmers as saying that the land will produce nothing without high doses of chemicals. The soil, they say, has become hooked on drugs.

One of cotton's victims is the Aral, once the world's fourth-largest inland sea. Its waters have been diverted to irrigate the cotton fields, and it is dying. More than 60 percent of its volume is gone, and most of its fish have disappeared. About 75,000 tons of salt and dust are sucked into the atmosphere each year from its dry bed and dumped across a broad swath of Uzbekistan and Turkmenistan, with disastrous effects for farming.

Other seas and lakes are threatened, environmental activists say. These include the Azov Sea in the south, Lake Ladoga north of Leningrad and Siberia's deep Lake Baikal, which holds one-fifth of the world's fresh water.

In the far west of the Soviet Union, a multilayered series of environmental threats hangs over the industry-choked Ukraine.

``Forget Chernobyl for a moment,'' said Yury Shcherbak, a medical doctor, member of the Supreme Soviet and chairman of the republic's activist Greens.

``Imagine that there's a state in the United States that has 3 percent of the country's land mass but 25 percent of its industrial capacity - nearly all of its heavy industry, such as coal and steel, inefficient and highly polluting. Imagine that it exports energy but keeps the pollution for itself. And finally imagine that 60 percent of the land had been overplowed and is eroding.''

Add to that the world's worst nuclear accident, Shcherbak concludes, and you'll get a picture of the state of the Ukraine.

In nearly every part of the non-Russian Soviet republics, environmental activism was a prologue and perhaps a testing ground for more overtly nationalist actions.

All the demonstrations had a common implicit theme: Moscow made the plans for industrial development, the local republics paid the price of development, and Moscow took nearly all the profits. The theme became a rallying cry for regional autonomy movements now tearing at Soviet unity.