Texaco announced plans yesterday to drill for natural gas and oil in Charles County in Southern Maryland, an exploratory effort environmentalists immediately attacked as a threat to the Chesapeake Bay.

The proposed test well would be south of La Plata on three of the 30,000 acres under which Texaco leases mineral rights in Charles County. The test well would be 10,000 feet deep and would be near Pope's Creek, a mile and a half from the Potomac River.

Maryland and Virginia laws prohibit drilling in the waters of the bay or any of its tributaries, but there is no such ban on land drilling.

Texaco's plan will require a raft of state and county approvals, and it drew immediate opposition from the Chesapeake Bay Foundation, a leading environmental group that has opposed similar efforts in the past.

"Oil and gas drilling are extremely disruptive to natural habitat, and the Chesapeake Bay is the most fragile marine environment," said foundation President William C. Baker.

Texaco officials said they are more likely to find natural gas than oil in Southern Maryland, although they did not rule oil out altogether.

Since 1985, energy companies have expressed interest in exploring the so-called Taylorsville Basin, which extends from the Northern Neck of Virginia across the Potomac River through Charles County. Texaco and Exxon unsuccessfully drilled for oil and natural gas in Westmoreland County, Va., in 1989. Last spring, Exxon bowed out and assigned its Charles County leases to Texaco.

Before Texaco and Eastern Virginia Gas Go., its partner in the joint venture, can proceed, they must obtain zoning, grading and sanitation permits from the county, and a total of seven permits from two state agencies. In addition, the drilling must be approved by the Chesapeake Bay Critical Areas Commission if it is within a certain distance of tidal waters.

Texaco officials said they believe the test site is outside any such critical area. They said they hope to obtain the necessary permits and begin drilling at the site, now an unplanted field, late next year.