Swimmers seeking relief from the summer heat by plunging into the Chesapeake Bay may emerge with a yelp because the jellyfish infestation of Bay waters is now at its highest level in 12 years.

David Cargo, a marine scientist with the University of Maryland's Solomons Laboratory, yesterday blamed the relatively dry summer as being chiefly repsonsible for the flourishing jellyfish population, and described the situation as "shaping up at least as bad as 1969," something of a benchmark year for the nuisance created by the nettles.

Cargo described 1969 as a "100 percent year," meaning that the chances of a swimmer's being stung in the Bay by a jellyfish were virtually 100 percent.

Longtime residents of the bay area say that jellyfish are most abundant after a long, hot dry spell. "The water becomes more saline in the dry heat," one explained. "That allows the jellyfish to come further in towards the shore."

The Chesapeake jellyfish population is expected to peak in late August and September. Until then, swimmers should realize that swimming in the bay may be accompanied by a burning rash provoked by jellyfish stings, marine experts said.

The translucent, tentacled creatures, also known as "Medusas" and "sea nettles," sting with tiny elongated cells, or "hairs" on their tentacles. The cell releases toxins on contact with a foreign body. Jellyfish live on small organisms, which they paralyze with the sting. As lobster and crabfishermen know, a jellyfish infestation can deplete the shellfish larvae population, diminishing the next season's catch.

In some rare cases, says Dr. Toby Litovitz, a toxicologist who heads Georgetown University's poison center, jellyfish stings may provoke an allergic reaction, which can be fatal. For victims of jellyfish stings who feel dizzy and weak, with muscle pain, headache or spasms, Litovitz suggests a visit to a doctor or hospital.

For relief from the burning, painful rash provoked by a normal jellyfish sting, he recommends applying either meat tenderizer containing papain, which breaks down the protein in the jellyfish toxin, or a mild solution of household ammonia in water. These should be applied "before attempting to remove any adhered jellyfish tentacles," Litovitz warned.