A long-lost symphony by Mozart will have its U.S. premiere in the Kennedy Center on July 8, during the second of the Mostly Mozart Festival concerts to be heard there between July 7 and 11.

It is the symphony now officially listed as K. 19a in F Major. Mozart wrote it at the age of 9 in the spring of 1765 while he was visiting London with his father.It was during that visit that Mozart began to write symphonies. His first was Kochel (K.) 16 in E Flat.

The newly discovered symphony turned up in a pile of about 100 manuscripts given this spring to the Bavarian State Library in Munich by an anonymous donor. Dr. Robert Munster, director of the library's manuscript division, identified the handwriting as Mozart's. The F Major Symphony, the authenticity of which seems well established, is Mozart's third symphony.

Two other symphonies written before the new discovery and once attributed to Mozart are no longer included in the official list. One of them, K. 17 in B Flat, is dubious, and K. 18 is known to be the young Mozart's copy of a symphony by Carl Friedrich Abel, a contemporary of Johann Christian Bach.

Mozart admired and studied the music of both Abel and the famous son of Sebastian Bach, and his second symphony, K. 19, shows a decided development in youthful style. If these early symphonies give no hint of the symphonist Mozart was to become, they very quickly attain a level remarkable for any 9-year-old.

Leonard Slatkin will conduct the 14-minute symphony with the Mostly Mozart Festival Orchestra. The soloist that evening will be pianist Alicia de Larrocha, playing the C Minor Concerto. The program will close with the last of Mozart's symphonies, No. 41 in C.