Only one student in Washington's public schools - the lowest number ever - has qualified as a semifinalist in this year's National Merit Scholarship competition.

The high-scoring senior, Geoffrey V. Waterhouse, attends Wilson High School in upper Northwest. Last year Wilson produced five of the six semifinalists that attended D.C. public schools.

By contrast, the city's private schools have 36 Merit semifinalists this year, even though their high school enrollment is less than a quarter that of the public schools.

In the suburbs, Fairfax County has 138 semifinalists, 42 percent of all those in Maryland.

Thirty of the Montgomery semifinalists attended Walt Whitman High School in Bethesda, which has led all Washington area schools in the Merit scholarship competition since 1972. This year Whitman has the sixth largest number of Merit semifinalists of any high school in the nation.

Manhattan's Stuyvesant High School and Phillips Exeter Academy in New Hampshire led the nation with 53 semifinalists each.

The semifinalists from Whitman include two sisters, Geeti, 17, and Malini Bawa, 16, whose parents were born in India and who are naturalized Americans.

The students were picked on the basis of a 100-minute mutiple-choice examination in English and mathematics given last fall to about 1 million high school juniors enrolled in about 18,000 schools.

The test is similar to the Scholastic Aptitude Test (SAT), sponsored by the College Entrance Examination Board.

Besides Montgomery County's Whitman, six other schools - Langley and Woodson (14 each) and Lake Braddock and Madison (12 each); one Montgomery public school - Winston Churchill - and one Montgomery private school, Landon, each of which has 10.

The National Merit Scholarship Corporation, which has conducted the annual competition since 1955, warns that results should not be used to measure the quality of schools. But the list of semifinalists does indicate the distribution of exceptionally bright students.

Throughout the county, about 15,000 students have been named Merit semifinalists, a figure that represents about 0.5 percent of this year's high school seniors. Next spring about 4,100 will be chosen for National Merit scholarships.

The number of semifinalists named for any state or the District of Columbia is proportional to the size of its high school graduating class. Thus, although the same test is taken nation-wide, the lowest qualifying score differs from state to state. This year it is 200 points (out of a possible 240) in D.C., 198 in Maryland, and 196 in Virginia.

In addition to the student from Wilson High, the list of Merit semifinalist includes one student, Andrew Stumpff, who attended Capitol Page School last year. The school is operated by D.C. public school system for pages who work for the U.S. House and Senate.

Stumpff now has returned to his home-town high school in Warrensburgh, Mo.

The Price George's public schools have 27 Merit finalists this year, with the largest number, six from Eleanor Roosevelt Senior High in Greenbelt, Arlington County has 17 semifinalists, including seven from Washington-Lee High School and six from Yorktown, Alexandria has five, all of whom attend T. C. Williams, the citywide school for 11th and 12th graders.

Among Washington private schools those with the most semifinalists are St. Alban's (9) and Sidwell Friends (8).

A list of area semifinalists appears in the Weekly section of today's Washington Post .