The 354-room, 16-story Bethesda Marriott Hotel opened Wednesday in the Pooks Hill area, and Marriott is expected shortly to announce plans to start a 401-room hotel in the Tysons Corner area.

The Bethesda Marriott is the firm's first hotel in Montgomery County. The one planned for a site on the Leesburg Pike would be the fifth in Northern Virginia. Others are operating at Crystal City, Twin Bridges, Key Bridge and Dulles Airport.

In addition, Marriott will operate its first District-based hotel beginning late next year on a site being developed in conjunction with the revamping of the Blackie's House of Beef site at 23d and M Sts. NW.

In addition, a Marriott hotel also is scheduled to be part of the Pennsylvania Avenue redevelopment plan for the block bounded by 13th, 14th, E and F streets, and construction may be started in 1980. Marriott is a co-developer with Quadrangle development Corp. and the James W. Rouse Co. The project includes demolishing the Munsey Building and preserving the National Theater.

The Bethesda Marriott stands on a site where a castle once was located, when the 150-acre estate was owned by Merle Thorpe. CAPTION: Picture, The new Marriott at Wisconsin Avenue and the Beltway. By Gerald Martineau-The Washington Post