The National Symphony Orchestra has hired 15 new players for the season beginning next week, including a 24-year-old Japanese-born woman as the new principal flute player.

The additional of eight new positions brings the orchestra to a membership of 103, making it comparable in size for the first time with ensembles like the Boston, Chicago and Vienna orchestras.

Toshiko Kohno was among 28 players who were invited Aug. 21 to audition for the flutejob, which opened this season when conductor Mstislav Rostropovich fired veteran first-flutist Wallace Mann after an arbitration process.

Auditions are conducted by the members of the section in which there is an opening. After they pared down the list of candidates, Rostropovich made the final decision.

Kohno comes here from the Montreal Symphony, where she was an associate principal. In her five years of orchestral playing, she has never before been principal flute.

"I started very late," she observed yesterday. "It was only a little more than 12 years ago. But once I got started I got caught up in it and it moved along like a fire."

She is a protege of the Boston Symphony's noted first-flutist Doriot Anthony Dwyer, who described Kohno's style yesterday as "a combination of refinement and passion."

The only other principal chosen is Milton Stevens, now first trombone of the Denver Symphony. He replaces John Marcellus, who has accepted the trombone professorship at Rochester's Eastman School.

Proportionally, the biggest impact on the sound by the additional performers should come in the string bass. With the additional of two double basses the National Symphony will have 10, as many as any American orchestra. There are also two additional violins, viols and cellos.

The new violinists are Terrie Baune, a graduate this year of Oberlin; Holly Hamilton, from the New Haven Symphony; Hyun-Woo Kim, a Juilliard student; Ernestine B. Shor, a former National Symphony member from 1953 until 1956, when she joined the New Orleans Symphony, and Linda Schroeder, from the New Orleans Symphony.

The two new violists are Lynne Edelson, who has played most recently with the Aspen Festival Orchestra, and Miles Hoffman, a recent graduate of Yale and Juilliard.

David Budd, from the Rochester Philharmonic, is one of the new cellists, and the other is Yvonne Caruthers of the Buffalo Philharmonic.

The bass players are Arti Smith of the Minneapolis Orchestra, and John Hood of the North Carolina Symphony.

Assistant principal horn will be Steven Gross, who has performed with the Boston Symphony. The other addition to the horn section is Jay Wadenpfuhl, from the Fort Worth Symphony.