The share price of the Johannesburg Stock Exchange, the largest bourse in Africa, has fallen 15% in its first week trading as a public company.

The 119-year-old exchange, which is dominated by mining companies including BHP Billiton and Anglo American, floated last Monday under the name JSE Limited and is the only exchange in the country. It opened at R24.25 a share but by Friday had fallen to R20.90.