BOSTON, DEC. 19 -- Scientists at the New England Aquarium said today that a naturally occurring poison commonly known as red tide was responsible for the deaths of at least nine whales found washed ashore on Cape Cod beaches since Nov. 28.

The discovery marks the first time that red tide has been directly linked to the death of large marine mammals. And researchers think that the new information could help explain the deaths of hundreds of bottle-nosed dolphins along the mid-Atlantic coast last summer.

"This event may have given us a real clue," said Joseph R. Geraci, the consulting marine pathologist at the aquarium and the director of a federally sponsored study into the dolphin deaths.

Over the last three weeks, seven humpback whales and two minke whales were found beached, an occurrence that baffled and alarmed scientists here who track the migrating whale herds. Both humpbacks and minkes are endangered species.

Only about 450 humpbacks feed in New England waters during the summer months. And humpbacks normally are found washed ashore at a rate of just one every 12 or 18 months, Geraci said.

Researchers from several organizations were called in by Geraci to help determine what was killing the whales. But their efforts were hampered by decomposition.

On Wednesday, however, investigators spotted a humpback struggling in Cape Cod Bay, and researchers were able to perform an autopsy shortly after it died.

Red tide, also known as paralytic shellfish poisoning, was found in the internal organs of Atlantic mackerel that the whale had eaten, leading researchers to think that all of the dead whales had been feeding on poisoned mackerel.

Don Anderson of the Woods Hole Oceanographic Institute said red tide poison is produced by microscopic plants and is probably unrelated to any man-made pollutant. "It's a very natural phenomenon," he said.

However, Anderson also said the poison usually is produced in warm weather and is most often found in shellfish. "We have a bit of a mystery as to where this toxin might have come from," he said.

Because the toxin was found in the organs of mackerel, Anderson speculated that the red tide poisoning may have been acquired by the fish in a different area at an earlier date.

The Massachusetts Department of Public Health has issued an advisory warning against eating the fish, although officials noted that the poison had not been found in the flesh of the fish, and that levels of toxicity of the organs were below those that affect humans.

Red tide symptoms include a tingling of the tongue and lips and can lead to respiratory difficulties and, in rare cases, death.

Ralph Timperi of the state health department said that no cases of human red tide illnesses have been reported.

The discovery that red tide poisoning killed the whales here will lead investigators to reexamine the remains of dolphins washed ashore last summer in South Carolina, Georgia and northern Florida, Geraci said.

Earlier this year, investigators examining the dolphin remains found no trace of paralytic shellfish poisoning in either the flesh or liver of the mammals, and then discounted red tide as a cause of death.

But last week, investigators here also were unable to find traces of the poison in the flesh or liver of the dead whales, which the scientists say indicates that red tide can kill marine mammals without becoming concentrated in flesh or liver.

"It may not leave a trace in the animal's tissue once the food has been digested," Geraci said.

Investigators plan to go back to the dolphin remains and examine stomach contents, which were not tested for red tide poisoning. The new dolphin examinations, however, could prove inconclusive. "We have very little in the way of stomach contents," Geraci said.