Scientists said the new work was particularly significant because it used detailed genetic analysis, similar to the techniques of DNA fingerprinting, to prove that the 30-acre fungus was a discrete being, which had grown over the years by sending out clonal shoots of itself. Other extremely large fungal growths have been identified in the past, but researchers could never be sure that the growths represented individual fungi, rather than populations of smaller molds whose edges had become smeared together.

"We used genetic markers to distinguish between these two possibilities," Dr. Anderson said. "It shocked us to have found such a large fungal entity that is so ancient."

"A lot of people have asked us if this is an April Fool's joke," he continued. "I've assured them it is not." Larger Growths Are Possible

As startled as they were to discover the colossal patch of fuzz, the researchers said their Armillaria may not even be the largest fungal clone around.

"It's the most successful one we're aware of, but this is in a mixed forest with many kinds of trees," Dr. Bruhn said. "We would think where there was a stand of pure trees like birch or aspen, a single fungus might be more successful still." In that case a fungus with a taste for a particular type of tree might be able to proliferate especially quickly and over the entire area before encountering any competitors.

The new discovery also underscores the ubiquity and power of the planet's fungi, a kingdom of organisms quite distinct from the plant and animal kingdoms.

"Fungi are the base of all terrestrial ecosystems," Dr. Bruns said. "No ecosystem on the planet would continue to operate without fungi to decompose and recycle wood and plants."