WHEN DUTY CALLS, Frederic follows. Duty can be a harsh taskmaster, and in The Pirates of Penzance her demands contravene both conventional Victorian morality and the urgings of the heart. But what is poor Frederic to do? Given a half-deaf nursemaid who apprentices him to a pirate instead of a pilot until he is 21 years of age and a birthday which falls with inconvenient quadrennial regularity on Leap Day, he acts as any Gilbert and Sullivan character worth his salt is bound to: he follows every absurd proposition out to its invariably illogical conclusion.

Composed on the heels of HMS Pinafore, The Pirates of Penzance is one of the best known of the Gilbert and Sullivan canon. The show has very little dialogue; there's nothing here, for instance, to rival the verbal pyrotechnics between the two peers in Iolanthe or the pompous flatulence of Poo-Bah in The Mikado. Pirates' fame derives rather from its score, which is a typical G&S mix of rousing chorus numbers, patter songs and take-offs on Italian grand opera.

Duty can be frivolous as well as stern, and it is a pleasant enough to report that the production of The Pirates of Penzance now at the Agassiz is nothing short of triumphant. It is no paradox that the Gilbert and Sullivan Players present the very best in Harvard theater with admirable consistency: they draw consistently on the same, very talented mix of regulars to play analagous parts in show after show.

In P.D. Seltzer the Players have found a director who knows what to do with all that talent. Seltzer's version of Pirates boasts plenty of directorial business: There are the banners proclaiming "Death and Slaughter" and "Glory and the Grave," unfurled as the policemen prepare to combat the pirates; the purposeful delay in starting Major General Stanley's famous patter song ("I am the very model of a modern Major General"); the instantaneous characterization of the last policeman as a bumbler out of step with the rest. But more impressive is Seltzer's general handling of the cast--not only the leads, most of whom could do this sort of thing in their sleep by now, but the two choruses, whose members take on a life and individuality of their own.

Seltzer has ample material to work with. Take Tom Fuller. Having played just about every male lead in recent G&S history, from Ralph Rackstraw to Nanki Poo, Fuller last year went backstage to direct a first-class Iolanthe. Now he's back as Frederic, the pirate apprentice, and he's even better than ever. His mellow tenor ably navigates the vagaries of Sullivan's music, and the expressions of dolefulness and misgiving his face so readily assumes are perfect for the benighted slave of duty.

Susan Willets Van Cott, as Mabel, is good enough to play opposite Fuller. Van Cott, who previously portrayed Phyllis in Iolanthe, sings the lovely "Poor wandering one" with operatic ease, her voice blending with the orchestral accompaniment. The part of the Major General's daughter and Frederic's wife-to-be call for little dramatic range, but Van Cott at least walks through the role without woodeness.

Dennis Crowley, back once again to sing the patter song, is another familiar face. Crowley has somewhat less to do as Major General Stanley than he did as the Lord Chancellor in Iolanthe, but he carries the part off with the same Gilbertian quizzicality that has marked his previous successes. His rendering of the difficult patter song is excellent, with every word--well, just about every word--clearly audible.

One of the reasons this version of Pirates is so good is a lack of egregious weaknesses among the leads. Patty Woo is suitably mournful as Frederic's former nurse, and John Corenswet, as the Sergeant of Police, sings the famous "A policeman's lot is not a happy one" with a straight-faced sincerity that makes his an appropriate foil to the zany police chorus. Of all the major performances, only Gregrey Gorden's Pirate King sounds a sour note. Gorden lacks the bellowing bass and comic belligerence to sustain his caricature of the English peer gone wrong. Perhaps some fiercer make-up would have helped.

Gorden may be disappointing, but his pirate band is as colorful and dashing a crew of ne'er-do-wells as you could wish for. The non-encounter encounter between the pirates and the policemen in Act II, with its inspired choreography (three encores worth) culminating in a kickline of sworn foes dancing arm-in-arm, is the highlight of the show. As usual in Gilbert and Sullivan, the female chorus is not as interesting, but under Seltzer's direction, the Major General's enticingly attired daughters sing and dance in neat unison.

THE FIRST ACT of Pirates is by far the less exciting of the two. With Frederic's discovery in Act II that he is still at least morally, if not legally, bound to the pirates, the real dramatic complications begin, and one splendidly choreographed number follows another in stunning procession. The whole takes place against the backdrop of Roger Bardwell's sets--effective recreations of a rocky sea coast and a ruined chapel--and under the glare of Will Durfee and Steuart Thomsen's dramatic lighting. John C. Beichmann's orchestra, which keeps the cast company, is adequate, though not as impressive as past G&S ensembles.

There are G&S aficionados who delight in humming Sullivan's airs and are apt to break into a patter song at a moment's notice, or without it. Then there are those whose exposure to the masters of Victorian operetta has been painfully shoddy. The current G&S production of The Pirates of Penzance is bound to convince both groups that it is, it is after all a glorious thing to be a Gilbert and Sullivan fan.