A long-simmering scientific controversy over Pfiesteria piscicida, the microscopic cell suspected of killing fish and making people sick, reached the boiling point yesterday when five North Carolina scientists challenged the work of the nation's main Pfiesteria expert.

The scientists, led by biologist Wayne Litaker of the University of North Carolina, published the results of their research, saying they believe Pfiesteria is not the complicated creature described by JoAnn Burkholder of North Carolina State University, the organism's co-discoverer.

Abandoning the normal polite tone of scientific debate, the article called Burkholder's description of Pfiesteria "spurious." Litaker said Pfiesteria may not be the source of a toxin that has caused fish kills and human illnesses in Maryland and North Carolina.

"Toxic Pfiesteria life cycle stages that don't exist can't be toxic," Litaker said in a statement. And university spokesman Leslie H. Lang said, "This is the nail in the coffin for Pfiesteria."

But other scientists said the new study's findings do not outweigh the accumulated evidence of Pfiesteria's toxicity.

"It's detail, scientific detail," said Donald Boesch, president of the University of Maryland Center for Environmental Science. "It is an interesting scientific question," he added, but "these findings are irrelevant to the public's concern about whether this organism poses a risk to fish or humans."

The dinoflagellate has been blamed for fish kills and cases of human illness on lower Eastern Shore rivers in 1997. It has been found in most Chesapeake Bay rivers and creeks. Several studies suggest it is usually benign, but becomes toxic in warm, salty waters loaded with algae and schools of oily fish.

Burkholder found Pfiesteria has 24 different life stages, some of which are unique, and quickly shifts from toxic to nontoxic forms. Others have verified her key findings.

Litaker and his colleagues said they tried for two years, but were not able to get the single-celled algae to form bloblike amoebae, which is one of the unique, potentially toxic forms Burkholder has described.

Burkholder and other experts said Litaker and his co-workers were looking for the toxin at the wrong stage in Pfiesteria's life cycle. Though the amoebae stage has occasionally been toxic, it's not the most dangerous form, Burkholder said.

Also, some strains of Pfiesteria never produce a toxin, and the UNC-led team apparently didn't realize they were studying a nontoxic strain, she said.

In an interview, Litaker said some of Pfiesteria's life stages may be toxic, even if the one he studied is not. "It's a very open question," he said.

David Goshorn of the Maryland Department of Natural Resources said some are "losing sight of the forest for the trees."

If Pfiesteria isn't toxic, "it would be tough to explain what we have seen," said Goshorn, who directs DNR monitoring of rivers prone to Pfiesteria outbreaks.

"I don't see anything here that would cause us to change what we're doing," Goshorn said. "The big picture is still the same."