As many as a half-million fish turned up dead in a skinny tidal creek on Virginia's lower Eastern Shore and on the Pocomoke River early yesterday, in what officials are calling the largest fish kill around the Chesapeake Bay in at least a decade.

Maryland officials found the fish on Bullbegger Creek, which feeds the Pocomoke. Two summers ago, outbreaks of the toxic microbe Pfiesteria piscicida killed some 30,000 fish in the same area and prompted Maryland to close the Pocomoke as a health hazard.

The dead fish were identified as menhaden--the small oily fish that also fell prey to the pfiesteria outbreaks. But state biologists all but ruled out the microbe as the cause of yesterday's kill, blaming it instead on too little oxygen in the water, a condition they said may have been exacerbated by the season's enduring drought.

None of the dead fish showed lesions, which are considered an indicator of pfiesteria, said David Goshorn, a water quality expert with the Maryland Department of Natural Resources.

A state biologist took oxygen readings and found them extremely low--not an unusual occurrence on the lower Eastern Shore, where an abundance of poultry manure and other fertilizers washing off fields has been blamed for nurturing great blooms of oxygen-choking algae.

"It's not unusual for a school of fish to go up one of these creeks and basically suffocate themselves," Goshorn said.

The drought didn't help, he added. Less rain spilling into creeks means higher salinity in the water. Saltier water tends to spawn greater concentrations of algae, which suck oxygen out of the water. Meanwhile, heat limits water's ability to hold oxygen.

Though pfiesteria is not being blamed for yesterday's kill, officials took water samples yesterday and sent them to a laboratory in North Carolina to be tested for the microbe.

Jack Howard, a local waterman widely credited with first bringing attention to pfiesteria in 1997, said he spotted "a small number," perhaps dozens, of dead fish floating in the lower Pocomoke in Maryland waters Sunday. On Monday, he tipped off a Maryland biologist who was surveying the river.

Early yesterday, a Maryland Department of Natural Resources biologist was on the water, checking out Howard's reports.

He found a few dead fish, perhaps 100, below Shelltown in Maryland waters, said a department spokeswoman, Liz Kalinowski.

Then he explored farther, turning up Bullbegger Creek, a narrow, marshy waterway on the Virginia side of the Pocomoke.

There it was: a thick mass of floating fish. He followed the kill as far upstream as his boat could go--about a mile.

He was later joined by a team of state scientists, including Charles Poukish, the designated "fish kill response person" for the Department of the Environment. Poukish estimated the dead at a half-million. Ninety percent were concentrated in Bullbegger Creek. The rest were scattered through the main stem of the Pocomoke. The fish appeared to have been dead for as long as 18 hours, Poukish said.

"We haven't had a fish kill of this magnitude since about 1989," Poukish said.

Maryland officials reported the kill to their Virginia counterparts. A spokesman for the Virginia Department of Environmental Quality said the agency plans to send a team of scientists to the scene this morning to survey the damage. Maryland officials plan to return to the river tomorrow, as well.