A stone slab found in the state of Veracruz in Mexico bears 3,000-year-old writing previously unknown to scholars, according to archaeologists who say it is an example of the oldest script ever discovered in the Western Hemisphere.

The order and pattern of carved symbols appeared to be that of a true writing system, according to the scientists who have studied the slab. It had characteristics strikingly similar to imagery of the Olmec civilization, considered the earliest in the Americas, they said.

Finding a heretofore unknown writing system is a rare event. One of the last such discoveries, scholars say, was the Indus Valley script, identified by archaeologists in 1924.

The inscription on the stone slab, with 62 distinct signs, some of them repeated, has been tentatively dated to at least 900 B.C., and possibly earlier. That is 400 years or more before writing had been known to exist in Mesoamerica, the region from central Mexico through much of Central America — and by extension, to exist anywhere in the hemisphere.

The inscribed stone slab was discovered by Maria del Carmen Rodriguez of the National Institute of Anthropology and History of Mexico and by Ponciano Ortiz of Veracruz University. The archaeologists, who are husband and wife, are the lead authors of the report of the find, which will be published today in the journal Science.

Experts who have examined the symbols on the stone slab said they would need many more examples before they could hope to decipher them and read what is written. It appeared, they said, that the symbols in the inscription were unrelated to later Mesoamerican scripts, suggesting that this Olmec writing might have been practiced for only a few generations and may never have spread to surrounding cultures.

Stephen D. Houston of Brown University, a co-author of the report, acknowledged that this was a puzzle, and would probably be emphasized by some scholars who question the influence of the Olmec on the course of later Mesoamerican cultures.

But Houston called the find tantalizing, saying, "It could be the beginning of a new era of focus on the Olmec civilization."

Other participants in the research include Michael D. Coe of Yale; Karl A. Taube of the University of California, Riverside; and Alfredo Delgado Calderon of the National Institute of Anthropology and History.

Mesoamerica researchers who were not involved in the Veracruz discovery agreed that the signs appeared to be a true script, and that the slab could be expected to inspire more intensive study of the Olmecs, whose civilization emerged about 1200 B.C. and had all but disappeared by 400 B.C.

In an accompanying article in Science, Mary Pohl, an anthropologist at Florida State University who has excavated Olmec ruins, was quoted as saying, "This is an exciting discovery of great significance."

A few other researchers were skeptical of the dating of the inscription, noting that the stone was uncovered in a gravel quarry where it and other artifacts were jumbled and may have been out of their original context.

The discovery team said that ceramic shards, clay figurines and other broken artifacts accompanying the stone appeared to be from a particular phase of Olmec culture that ended about 900 B.C. But they acknowledged that the disarray at the site made it impossible to determine whether the stone had originally been in a place relating to the governing elite or to religious ceremony.

Richard A. Diehl, a specialist in Olmec research at the University of Alabama and another co-author of the report, said, "My colleagues and I are absolutely convinced the stone is authentic."

The stone slab first came to light in 1999, when road builders digging gravel came across it among debris from an ancient mound at Cascajal, a place the archaeologists called the "Olmec heartland." The village is on an island in southern Veracruz about a mile from San Lorenzo, where ruins have been found of the dominant Olmec city, which stood from 1200 B.C. to 900 B.C.

When the stone surfaced, Rodriguez and Ortiz were called in, and quickly recognized the potential importance of the find.

Only after six years of further excavations searching for more writing specimens, and comparative analysis with previously known Olmec iconography, did the two archaeologists invite other Mesoamerica scholars to join the study earlier this year. Though some other reported examples of Olmec "writing" in recent years failed to stand up to scrutiny, the team concluded that the Cascajal stone, as it is being called, was the real thing.

The tiny, delicate symbols are incised on the concave top surface of a block of soft stone that measures about 14 inches long, 8 inches wide and 5 inches thick.

Houston, who was a leader in deciphering Maya writing, examined the stone looking for clues that the symbols were true writing and not just iconography unrelated to a language. He said in an interview that he detected regular patterns and order, suggesting "a text segmented into what almost look like sentences, with clear beginnings and clear endings."

Some of the pictographic signs were frequently repeated, Houston said, particularly ones that looked like an insect or a lizard. He suspected that these might be signs alerting the reader to the use of words that sound alike but have different meanings — as in the difference between "I" and "eye" in English.

All in all, Houston concluded, "the linear sequencing, the regularity of signs, the clear patterns of ordering, they tell me this is writing. But we don't know what it says."