In one corner there is The Fix. In the other corner there is Beauty and the Beast. One is a new musical at the 250-seat Donmar Warehouse, which within a couple of months has gone from a demo tape that landed on Sir Cameron Mackintosh's desk to a snazzy little production that introduces the work of an unknown American composer-lyricist team. The other is a not-quite-so-new musical at the 2,200-seat Dominion, which costs pounds 10 million and has so far opened in nine other countries. Sad to report: last week David got slain by Goliath.

Any move in musicals away from the "mega" to the "mini" ought to be applauded, but The Fix, a rock musical with book and lyrics by John Dempsey and music by Dana P Rowe, chooses to shoot fish in a barrel. Presidential candidates, says this tame satirical piece, will just say anything to get elected.

A presidential candidate has a heart attack, and his formidable spouse - played by Kathryn Evans, with a hairdo that wouldn' t get ruffled by a hurricane - determines that if she is never going to be the president' s wife she will be the president' s mother. The Fix opens with the husband's funeral, and follows her son's rise, in a step-by-step way, as if all this political packaging was news to us.

The Fix uses a mix of styles and genres. Rowe' s music alternates between vaudeville, rock 'n'roll, country and western, and blues. Dempsey's plot starts with satirical two-dimensional portraits, and then shifts gear, asking us to treat these people more sympathetically. Some characters - the newspaper hacks, for instance, and the mafia in their dark suits, black shirts and white ties - look as if they have simply stepped out of other musicals.

In Rob Howell's designs, neon scenery flies in and out, and a TV screen broadcasts news bulletins. Howard Harrison lays on a lavish display of spotlights, and Charles Augins choreographs a chic finger-clicking chorus of security guards: in dark suits, shades, and radio mikes. But the experts have arrived too late.

As the candidate Cal, John Barrowman keeps reminding one of other actors: he has Hugh Grant's grin, Christopher Reeve's clean-cut looks, and Warren Beatty's schoolboy charm. He's a good singer in a part that we never know how to judge. But his role is credible alongside the sinister, scheming uncle Grahame, played by Philip Quast. Grahame has slicked back hair, a trim beard, and a stutter. He is a cripple, and a homosexual who forces his nephew to have sex with him. No worries about offending anyone there then. Not that the piece goes deep enough to rouse our indignation: the satire has a self-satisfied air. Sam Mendes's production - well-drilled and neatly packaged - displays a few too many of the qualities it sets out to attack.

Beauty and the Beast, with music by Alan Menken and lyrics by Howard Ashman and Tim Rice, is pantomime done on a spectacular scale. The worry was that Disney's big venture into the West End would be the theatrical equivalent of the opening of a new burger chain: McBeauty and McBeast.

But the director, Robert Jess Roth, cleverly bridges the gap between hi-tech production values - the special effects are pretty good - and the nostalgic appeal of a traditional fairy tale. Stanley A Meyer's set resembles a toy theatre with backdrops and flats, and cut-outs sliding in and out. The costumes by Ann Hould-Ward are wittily over the top. Matt West's choreography ensures that we get our money's worth.- particularly in the Busby Berkeley-style number where dancers rise up on a staircase of saucers with spinning plates attached to their backs while fireworks explode out of giant champagne bottles.

Alasdair Harvey is a bit drippy as the Beast - and the Prince. But Julie- Alanah Brighten is a sparky, independent Belle, who more than lives up to her character's name, not to mention her own. Derek Griffiths is funny and energetic as Lumiere, the French candle-holder, and Mary Millar deserves to be everyone's favourite aunt, as the tea-pot shaped Mrs Potts. Without the bombast that saddles some of the other blockbusters, this piece of theme-park theatre deserves to be around for a couple of years, but not - multiculturalists must hope - for much longer than that.

The Sixties classic, Marat/Sade by the German Peter Weiss, is a better play than it appears in Jeremy Sams's low-temperature revival at the National. Of course in Blair's Britain the revolutionary spirit that lay behind the early productions has turned into pragmatism. The actress who played Charlotte Corday in Peter Brook's 1964 production is now a junior minister at the Department of Transport.

This is the second of the two plays done at the Olivier in-the-round, and the new staging dissipates the atmosphere. Sams's production is neither stark nor detailed enough to draw us convincingly into the Charenton asylum in 1808. It always feels iffy watching actors playing people who are insane. Here, actors play asylum inmates acting roles from the French Revolution as a revolutionary piece of therapy. It isn't easy for them or us.

Weiss's play centres on an argument between the extreme individualism of the Marquis de Sade, a Charenton inmate, and the revolutionary zeal of Marat, scribbling away in his bath-tub, played by another inmate. A shiny-headed, thin-lipped David Calder catches the complicated depths of de Sade. But either Corin Redgrave or the paranoid he is playing has chosen a vacuous reading of Marat, rolling his eyes, sticking out his lower jaw and slurring his speech (with a Scots accent). It lets the steam right out of the debate. Only the orange-haired Anastasia Hille, tip-toeing round with trembling fingers outstretched, manages powerfully to fuse the character of the narcoleptic inmate with that of Marat's assassin, Charlotte Corday. The end is striking: the cast run amok, guards shoot in the air, and netting cascades down, enveloping the inmates. For the first time, there is a sense of entrapment.

The best show of the week was The Seagull. The virtues of Peter Hall's repertory company become clearer with each production. A genuinely high standard of acting runs right through the cast. The designs are subtle, spare, and serve rather than dominate the play. The emphasis is on ensuring clarity of performance.

In the programme Sir Peter notes that the essence of Chekhov's comedy is "total obsession with self". Tom Stoppard has provided a translation which is crisp and relaxed, allusive and moving. This is seized on by a dozen of the best actors in London, who each turn in their lively study in self-centredness.

Everyone is on form: Felicity Kendal as the fluttery actress Madame Arkadina, who can't bear her's son play, Michael Pennington as her lumbering lover Trigorin, David Yelland as the pessimistic doctor, and Greg Hicks as the beleaguered schoolmaster. The best performance comes from Victoria Hamilton in the role of the flighty, impulsive Nina, who collapses on the ground with relief moments after her first entrance. She has a lovely directness, not least when Dominic West, the intense young writer Konstantin, hands her a dead seagull: "Now what's that supposed to mean?" she asks. This is a very enjoyable Seagull, which answers many of those questions.

`The Fix': Donmar, WC2 (0171 369 1732) to 14 June; 'Beauty and the Beast': Dominion, W1 (0171 416 6060). 'Marat/Sade': Olivier, SE1 (0171 928 2252) to 21 June; 'The Seagull': Old Vic, SE1 (0171 928 7616) to June 7.