BATAVIA, Ill., March 2— Culminating nearly a decade of intense effort, two rival groups of physicists announced today that they had found the elusive top quark -- an ephemeral building block of matter that probably holds clues to some of the ultimate riddles of existence.

The announcements brought sustained applause and a barrage of questions from an overflow audience of physicists at the Fermi National Accelerator Laboratory, where the work was done. Fermilab has the world's most powerful particle accelerator.

The two competing scientific teams, each with about 450 scientists and each using a separate detection system, reported that after a long chase in which there had been several false sightings of the top quark, this monstrously heavy but elusive particle has finally been cornered and measured. The results of the two groups' independent measurements differed somewhat, but when margins of error were taken into account, the scientists agreed that the results were consistent.

One of the teams, the CDF Collaboration (standing for Collider Detector at Fermilab) reported last April that it had found evidence of the quark's existence. But at the time, the group lacked enough statistical evidence to claim discovery, and the competing group, the D0 (for D-Zero) Collaboration, which had even less evidence of its own, branded the CDF announcement as premature.

The achievement claimed today by both teams leaves virtually no room for doubt, however, and the discovery was hailed as a landmark in science. Hazel O'Leary, who as Secretary of Energy heads the Federal agency providing most of the money for research at Fermilab, called the discovery a "major contribution to human understanding of the fundamentals of the universe."

The finding confirms a prediction based on a theory known as the Standard Model that nature has provided the universe with six types of quarks; the other five, the up, down, strange, charm and bottom quarks had all been known or discovered by 1977. Since the infancy of the universe shortly after the Big Bang -- estimated at 10 billion to 20 billion years ago -- only the up and down quarks have survived in nature, and the protons and neutrons that make up the nuclei of all atoms are built from combinations of these two quarks; the other quarks disappeared from the observed universe, but have been recreated by modern particle accelerators.

Dr. Leon M. Lederman, a winner of the Nobel Prize in Physics and a former director of Fermilab, said at today's meeting that he doubted there could be any more quark types but that "we know there's a lot of dark matter out in the universe that we can't identify."

"We're still in for a lot of surprises," he added.

But more important than merely completing the table of quarks predicted by theory, the top quark may now begin to shed light on a deep philosophical question: everything in the universe, from the most distant galaxy to a rose petal, is made of quarks. Were the masses and other properties of these particles determined by random chance, or by some fundamental unifying plan? If so, what is that plan, and how might gravity, the least understood of the four forces of nature, be related to it?

"This monster, compared with all the other quarks, is like a big cowbird's egg in a nest of little sparrow eggs," said Dr. Paul D. Grannis, a leader of the D0 group. "It's so peculiar it must hold clues to some important new physics."

"The top quark has turned out to be so heavy," added Dr. John Peoples, director of Fermilab, "that it's kind of a laboratory in itself, from which many new experiments will certainly yield important insights."

It may be, scientists believe, that quarks (and the higher forms of matter they make up) are endowed with mass by interacting with an all-pervading universal "field," with which they communicate through a hypothetical particle called the Higgs boson. To find and measure the Higgs boson would be as exciting for a physicist as the creation of life in a test tube would be for a biologist.

One of the questions high-energy physicists regard as fundamental is whether there is a single type of Higgs boson, or several types. Theory predicts that if is possible to accurately measure the masses of two known particles -- the top quarks and the W particles that transmit the weak nuclear force -- it will be possible to determine whether there are one or more than one Higgs bosons.

"We're so elated by the discovery of the top quark that we haven't yet begun to sift all the data," said Dr. Boaz Klima of Fermilab, one of the leaders of the successful search. "But this particle is so astonisingly heavy that its decay may give us hints of a lot of other things, perhaps even of supersymmetric particles."

The quest for supersymmetric particles by the world's most powerful accelerators during the last decade has failed to turn up any evidence that they exist, but according to some theories, they may be so heavy they are beyond reach of present-day accelerators. If supersymmetric particles could be shown to exist, they might offer scientists a tool for learning how gravity is related to the other forces of nature: the electromagnetic force and the strong and weak nuclear forces.