THEATRE / Begging not to differ: Sarah Hemming on the London transfer of The Beggar's Opera; plus the best of the Fringe

Posters on the London Underground for the RSC's production of The Beggar's Opera (Barbican, EC2) describe it as 'The first GREAT British musical'. To anyone who knows that the director is John Caird, this offers a sizeable hint as to the production's approach. Caird worked with Trevor Nunn on the RSC's blockbuster hit, Les Miserables. In his treatment of John Gay's classic we might expect great energy, confidence and polish, sumptuous detail and a popular streak of sentimentality.

That is just what we get. Caird's production plunges us with relish into 18th-century London. Kendra Ullyart's busy warren of a set, composed of crazily tilting wooden walkways, ladders, ropes and dark holes, seethes with bodies at the snap of the beggar poet's fingers. The crowd is choreographed so that the whole set seems to creep with life. Throughout, the beggars linger in the crannies, providing entertaining 'off-stage' business on one level, serving as a reminder, on another, that solitude and space were rare commodities in the 18th-century London underworld.

There are some witty, hugely entertaining scenes: a luridly lit highway robbery is staged with great vigour and a couple of chairs; Macheath's evening with the local whores veers towards pantomime when one turns out to be played by an immense and humourless male beggar, and there's a false ending, when the poet is forced, to his chagrin, to bring Macheath back from the gallows.

The music is deftly arranged by Ilona Sekacz to combine authentic and modern instruments, and often sweetly sung (particularly Macheath and Polly's haunting love song 'Over the Hills and Far Away'), and the production is propelled forwards restlessly - every character seems eager to pack in as much as possible before death. This is particularly true of David Burt's dashing, energetic Macheath - a charming rogue in a hurry.

All the tumult is there, what is missing is the dark shadow. This is the beggar's opera - a play within a play - and therefore any sentimentality or raucous gaiety can be happily embraced. But it is also John Gay's bitter portrait of a rancid existence at the bottom of society, so the cruel reality needs somehow to jut through the surface jollity. This is poverty with polish.

From the Mississippi Delta (Cochrane, WC1) is also an account of life in poverty. Dr Endesha Ida Mae Holland's autobiographical account of growing up as a young black girl in the Deep South of the Forties was staged at the Young Vic three years ago and has now been revived by the same director Annie Castledine. The production's strength lies in the simplicity of the staging. Iona McLeish's set is a plain arrangement of rust-coloured corrugated iron that roughly suggests hardship. Against this simple set it is left to the three black actresses to carry the story, and they do so magnificently.

Pauline Black, Joy Richardson and Josette Bushell-Mingo share the scenes of Endesha's childhood, passing the characters between them and slipping into traditional spiritual and gospel songs to underline the stories. The play traces, in short episodes, the life of Endesha's brave mother and her death, at the hands of the Ku-Klux-Klan, and Endesha's own struggle to escape her background and get a university education. There's an uncomfortable touch of self-congratulation about the ending, but otherwise this is a moving piece, beautifully performed. Pauline Black brings dignity and grace to her roles, Joy Richardson brings wit. But the real delight is Josette Bushell- Mingo, who not only surprises with her impish ability to change character, but gives a piercing performance of 'Strange Fruit'. Such performances deserve more than a half-full house - where are the Cochrane's audiences?

In Stephanie McKnight's Beat the Air (Finborough, SW10), a downtrodden woman also takes on the system and wins - to some extent. Hope Macquarie's husband is a Scottish fisherman; when his boat disappears off the coast one day in completely calm conditions and yet no one can recover his body, she smells a cover-up. She abandons her life as a normal person and heads for London, determined to bludgeon an answer from the Ministry of Defence. Having taken up residence in a downbeat flat, she makes her assault.

McKnight's play is over- long, and weakest when it deals with the big issue - the combative meeting between Hope and the man from the ministry often drops into cliche. It is at its best when drawing the curious assortment of misfits in Hope's block of flats and the relationships between them, and these characters and scenes are warmly drawn in Sarah Frankcom's production.

'The Beggar's Opera' is in rep (071-638 8891); 'From the Mississippi Delta' runs to 1 May (071-242 7040); 'Beat the Air' runs to 24 Apr (071-373 3842)