Maryland officials yesterday advised people to use caution when boating, swimming or fishing on a creek feeding the Manokin River on the Eastern Shore after finding in the water small amounts of what appears to be the toxic microbe Pfiesteria piscicida, which sickened people and killed thousands of fish two summers ago.

Officials were alerted to the microbe when a person early last week began exhibiting skin irritations after being in Back Creek in Somerset County. Though the symptoms are associated with pfiesteria exposure, that case has not been confirmed, officials said. Two other people also reported exhibiting the early stages of pfiesteria exposure, and those cases are under investigation.

Microbes that appeared to be pfiesteria were in three water samples taken from the creek last week. In one sample, the amount of the microbe detected would be enough to injure or kill fish, but so far officials said they had not found any dead fish or ones with telltale lesions.

Department of Natural Resources police in patrol boats will advise people on the water to use caution on a 2 1/2-mile stretch of the creek between Raccoon Point and the Millard Long Road bridge.

"While there is no confirmation of toxic pfiesteria, Marylanders should use prudent caution in this part of Back Creek," Gov. Parris N. Glendening (D) said in a statement. "There have been no indications that this occurrence extends beyond this very small, remote, localized area."

Department of Natural Resources spokesman John Surrick said state biologists had been regularly checking waterways and in the last two weeks had seen only healthy fish in Back Creek.

They returned to the creek after the reports of the person with skin irritations and took additional samples, which were tested in two labs. Those samples turned up what appeared to be pfiesteria cells, Surrick said. Department of Natural Resources biologists will continue to monitor the creek, taking water samples and checking fish in the coming days, he said.

An outbreak of pfiesteria in August 1997 prompted Glendening to close several waterways leading to the Chesapeake Bay, ignited fears about the quality of seafood and prompted political battles in Annapolis over the source of the toxic microbe.

The political battles yielded new regulations on Eastern Shore farmers. Some scientists said pfiesteria thrives on the manure runoff from the millions of chickens raised on massive farms on the Delmarva peninsula. The manure routinely is used as fertilizer by crop farmers.

Under new regulations being phased in, farmers must limit the amount of manure and other fertilizers they use so that they do not leak into waterways.

There were no significant outbreaks of the toxic microbe last summer, but it was about this time in 1997 that pfiesteria prompted the closing of several waterways, including a five-mile stretch of the scenic Pocomoke River. The closures prohibited all fishing, swimming and use of personal watercraft on the waterways.

Watermen reported seeing gaping red sores on fish several months before the massive kills, in which thousands of menhaden, rockfish and croakers were destroyed.