On Thursday night, the creators of the Harvard Theater Workshop graduated into the Brattle Theater Company. Few Harvard commencements have been so successful and so entertaining.

Last year the group was amateur and called professional. This year it is professional and called first class for imagination, polish, and skill, but especially for imagination.

The test came on Shakespeare's "Twelfth Night," a well-tried. In last it is being tried again in New York this week. But you are lucky to be in Cambridge. The performance here is no chuke job. The costumes and get are extravagantly eighteenth century, and appearing prominently in gold braid and squashed top-hat is the late W. C. Fields via Jerry Kilty as Sir Tobey Belch. In this Kilty has resisted the case of playing another Falstaff, which he does well, and instead successfully innovates a double impersonation.

But even with the noise and nonsense of Kilty's syphon squirting and the hilarious performance of Thayer David as Sir Andrew Aguecheck, the production has more substance than the usual farce. Donald Stevens is a thoughtful and detached clown. While Robert Fletcher's griping, prissy interpretation of Malvolio excludes all customary pity for his plight, it does not justify the brutal treatment he receives from the fetching chambermaid, Jan Farrand, and her licentious colleagues, Sir Tobey and Sir Andrew.

Betty Field, the guest star, brings freshness and charm to the role of Viola. Her non-classical experience is an asset to the unorthodox production.

Robert O'Hearn's efficient and fascinating two-level stage tops the Company's previous triumphs in design, and it is hard to subordinate Richard Baldridge's beautiful staging to the play itself.

If you want some exciting and highly entertaining theater, disregard the window at Mandrake and don't miss the big beginning at Brattle Hall this week.