The proposed merger of the state's only county-run bus system for private school students with the public school system is being opposed here by parents with children in Catholic, Mennonite and Baptist schools who contend it could result in elimination of service to their children.

St. Mary's County is the only county in Maryland to maintain two separately funded and administered bus systems, with separate buses and routes for private and public students.

Now the county commissioners are considering merging the bus systems as a way to cut costs.

Many parents of the county's 2,500 Catholic school students are afraid such a move would be only the first step to eliminating busing entirely for all private school students, said Bishop Thomas Lyons, regional bishop for Prince George's County and Southern Maryland.

Parents are concerned that "the buses won't go near the Catholic schools and they won't have the money to provide their own transportation," he said.

He said that only about 2 percent of Prince George's 11,000 Catholic school pupils are able to make use of buses on existing public school routes there. "The rest pay $400 or $500 extra a year extra in transport costs," Lyons said.

County Administrator Edward V. Cox said no such wholesale dismantling of the private school system is planned. "No action has been taken. We are just beginning to study the issue," Cox said.

A public hearing was scheduled last night "to dispel such rumors and to get feedback from residents," Cox said. The controversy erupted earlier this month when a local newspaper reported that Cox had presented the county's five commissioners with a memo suggesting possible changes in the transportation system at a meeting closed to the public.

By early this week, the commissioners had gotten nearly 300 letters urging them to leave the bus system alone and requesting a public hearing. The bulk were form letters sent home by principals at the county's six Catholic elementary schools and one high school, Cox said.

One reason St. Mary's has maintained its separate systems is that state law strictly forbids the use of state money to finance transportation for private school students unless they are handicapped, said Maryland Department of Education spokesman Gus Crenson.

About half the counties in Maryland -- including Montgomery and Frederick -- have laws that require or permit transportation for private school students as long as the county picks up the tab, said Richard Alexander, chief of pupil transportation for the State Department of Education.