Orange County agriculture officials said Thursday that they have launched an investigation into a pesticide used on county crops after high levels of the insecticide were found in Newport Bay.

The inquiry comes 11 days before Newport Beach is likely to call for an immediate ban on endosulfan, a pesticide that city officials fear is harming the bay's mussels and clams.

"This can be very toxic and very harmful to the marine environment," said Jack Skinner, a member of the Newport Beach Harbor Quality Committee, the group that proposed the ban. "This is a danger sign that something is wrong."

Frank Parsons, chief deputy agriculture director, said that state health officials have agreed to aid in the investigation of the pesticide, which is used on 22 farms in Orange County--most of them located in the Irvine area north of Newport Bay.

Because of the potential health hazard posed by the pesticide, endosulfan is classified as a restricted material and can be used only with a permit issued by the county, Parsons said.

The inquiry will likely focus on why levels of the pesticide have risen over the last five years and what effect it has on wildlife.

Parsons said that once the investigation is completed, the county will determine whether it is necessary to stop issuing new permits or even revoke existing permits.

Concern over endosulfan concentrations stems from a report by the California Regional Water Quality Control Board that showed a dramatic increase in levels of the pesticide in the bay.

The Mussel Watch Program study, which measured the presence of endosulfan and more than 20 other chemicals, found up to a 2,000-fold increase in endosulfan levels on the marine life in the mid-1980s.

In 1985, for example, mussels found near the Newport Channel entrance had endosulfan concentrations that measured 4 parts per billion. In 1987, the levels jumped to 459 parts per billion.

Endosulfan concentrations in mussels living near the MacArthur Boulevard Bridge jumped from 44 parts per billion in 1985 to 2,574 parts per billion in 1988, according to Pat Carroll of the water quality board.

Pesticide levels for 1988 dropped in some portions of Newport Bay, but they did not return to amounts detected before 1985, Carroll said. Figures for 1989 are not yet available.

State water officials suspect the endosulfan creeps from the watershed beneath farmland into San Diego Creek, which runs into the bay, Carroll said.