The 40th anniversary season of Arena Stage got off with a bang and now is muddling along with a whimper. Its spectacular performance of Bertolt Brecht's "The Caucasian Chalk Circle" has been followed by a soggy one of Thornton Wilder's "Our Town," which proves nothing so much as that even the best of theatrical companies -- and Arena most certainly is among them -- is capable of putting on a bad show.

In part this is no fault of Arena's. Now more than a half-century old, "Our Town" is showing both its age and the flaws that excessive familiarity invariably exposes. Seeing it for the first time in at least a quarter-century, I was taken aback by the corniness of so much of its language, the banality of its rhetoric and the clumsiness of its construction; it's a period piece pure and simple, and its continuing presence in the standard repertoire probably has more to do with nostalgia than with genuine dramatic merit.

But Arena has compounded the play's problems. For whatever reason its director, Douglas Wager, has chosen to emphasize the sappiest aspects of Wilder's play; he has been abetted in this misguided endeavor by much of the cast, in particular Robert Prosky's gratingly aw-shucks interpretation of the role of Stage Manager. But a more serious problem is Arena's attempt to turn "Our Town" into a vehicle for the new political correctness. In what can only be viewed as a wildly inappropriate example of "nontraditional casting," it has turned this melodrama about turn-of-the-century white New England Protestants into a multiracial and multicultural extravaganza, in the process making it into something it simply is not.

It is an effort consistent with longstanding Arena policy as determined in large measure by Zelda Fichandler, the co-founder of the theater who is retiring at the end of the current season as its producing director. A tribute to her in the playbill now being distributed to Arena patrons reads in part:

"Her commitment has been instrumental in Arena's efforts to diversify its staff and to be more responsive to the significant minority populations that constitute its community. In keeping with her artistic vision to embrace cultural diversity, Zelda has led the theater in a far-reaching effort to increase multicultural representation on Arena's stages, in the audience, and among theater personnel."

This is an admirable policy and it has had many admirable results. Few cultural institutions in the country have been so responsive to American heterogeneity as Arena Stage; not merely was it among the first to stage plays that try to deal honestly with racial and ethnic matters -- it was at Arena, you will recall, that "The Great White Hope" made its famous debut -- but it has been a pioneer in offering employment to members of minority groups, blacks in particular, not merely in visible places on stage but in behind-the-scenes jobs as well.

Its commitment to nontraditional casting is part of this, and its production of "The Caucasian Chalk Circle" showed precisely how effective such casting can be. Brecht's play may contain an ample measure of twaddle, but it is full of theatrical energy and it transcends, in setting and cast of characters, most cultural and racial considerations. Arena's production turned the play into a spectacularly diverse human tapestry, in so doing emphasizing its essentially fabulous nature and making it into a better play than it really is.

But "Our Town" is quite another matter. To be sure it is, like "The Caucasian Chalk Circle," a fable, but it is one set in a very specific, very narrow time and place. If it speaks to the universal, as certainly it does, it speaks all the same through the medium of a New Hampshire Yankee town, in the language of the people who inhabit such a town and in customs such as would have been encountered there from 1901 to 1913, the years embraced by the play. Clearly one of the play's themes is that even in a place as remote and provincial and homogenous as Grover's Corners, we can find a microcosm of the world or, if you like, the universe; but its homogeneity is essential to its character, indeed may well be essential to Wilder's message, and to fudge it is to fudge the play itself.

This is what Arena does. Of the 26 members of its cast (several play more than one role), seven are black and one is Hispanic. This means among other things that there are two pairs of interracial siblings -- George and Rebecca Gibbs, Emily and Wally Webb -- and that the role of Dr. Gibbs is performed by an actor whose accent calls to mind not a turn-of-the-century Yankee physician but Ricky Ricardo. It goes without saying that this is the result of honorable intentions; the effect, though, is merely jarring and distracting.

There are two reasons for this. The first and most obvious is that it bears no relationship to reality, either Wilder's or New Hampshire's. As recently as 1980 that state had a black population of less than one-half of 1 percent, and a Hispanic population of .005 percent; New Hampshire, that is to say, even now comes about as close to being lily-white as any of the American states, so to represent it otherwise may be noble but it is also preposterous.

The second and in some respects more important reason is that in attempting to transcend race and ethnicity, this production inadvertently calls attention to both. The young actors playing Rebecca and Wally are attractive and able, but because they are black it simply defies credulity that they could be sister and brother to, respectively, George and Emily; the actor playing Dr. Gibbs does his best, but his Hispanic accent is wholly incongruous within a cast the other members of which are striving mightily to sound like New Hampshire Yankees.

The result is that the playgoer ends up making a conscious effort to put the cast's racial and ethnic identities out of mind, which gets in the way of the play and renders it, if not entirely implausible, in great measure so. To say this is not to denigrate nontraditional casting but to point out that there are some plays (and movies) in which it works and some in which it does not. Try to imagine the title role of "Purlie Victorious" played by a white actor, or the matriarch in "A Raisin in the Sun" by a white actress, and you get the point: The casts of certain plays have distinct racial or ethnic compositions, just as do certain families and communities in the world itself, and to represent them otherwise is to misrepresent them.

Nontraditional casting is a wonderful idea in principle and often in practice, as "The Caucasian Chalk Circle" so memorably demonstrated; as was noted in this space during the brief furor over the casting of "Miss Saigon," the theater has the power to make us disbelieve what our own eyes tell us, a power that nontraditional casting depends upon for its effectiveness. But it is one thing to open acting opportunities where in the past they have been closed, and another to impose artificial racial or ethnic or sexual constraints on plays to which they are ill-suited.

Like everything else in this over-sensitized society of ours, nontraditional casting is a tricky business. I don't propose to have the final answer to it, and I guess that if I had to choose between clumsy efforts to lower racial barriers such as Arena's "Our Town" and no efforts at all, I'd choose "Our Town." But the play now being performed at Arena Stage, however deep the bow it makes toward political correctness, is not the play that Thornton Wilder wrote; that should count for something, but these days it doesn't.