Concluding a yearlong search, William Stewart, a 50-year-old theater executive, has been named new managing director of Arena Stage. He will assume his duties as Arena's top administrator on Jan. 1.

Stewart, whose appointment was announced yesterday, will succeed Thomas Fichandler, who has held the post since the theater's founding in 1950 -- initially as a volunteer and then five years later on a full-time basis. Fichandler is scheduled to retire in the spring, although he has indicated his willingness to continue raising money for Arena in the future and to do "whatever else the new managing director may want me to do."

In his new position, Stewart will be responsible for the financial health of the theater and its critical fund-raising and marketing efforts at an annual salary described only as "well above $50,000." He comes from the Hartford Stage Company, where he was managing director from 1969 to 1973, and from 1976 to the present. In between, he was managing director of the American Shakespeare Theatre in Stratford, Conn. A lecturer at the Yale Drama School since 1974, he also served five years on the executive committee of the League of Resident Theaters.

While budgetary restraints are increasingly common in regional theaters, Fichandler has earned a nationwide reputation over the years as a financial administrator who puts theatrical goals first. "Tom has been my partner since Day One," said Zelda Fichandler, Arena's founder and Fichandler's former wife, yesterday. "Even in the early years when he was working weekends and evenings, he was the person with whom I shared everything along the way. He's often seemed to be in the background, but it was in the background that the structure was built and that the solidifying took place."

Calling Stewart "a central architect of our future growth," she described his philosophy as being "very much in tune with Tom's. Bill believes that the reason for his job is to nurture an environment in which the artist can thrive."

A major problem facing regional theaters today is finding a new generation to take over for those who, in many instances, founded the institutions. Fichandler hailed his replacement yesterday as "the right person" and said that Stewart's appointment promised an "orderly change in leadership that will keep Arena Stage at the forefront of the regional theater movement."