Stocks in clean-energy companies rebounded to reach all-time highs in the past two months, after dipping with the rest of the stock market earlier in the summer.

The WilderHill New Energy Global Innovation Index (symbol NEX on the American Stock Exchange) now stands at almost twice its level at the beginning of 2006 ? reflecting the new-found tendency of mainstream investors to take 'alternative energy' stocks seriously.

A closer look at the index shows a mixed pattern, says Robert Wilder, whose California-based consultancy WilderShares co-compiles the index with New Energy Finance of London.

In particular, investors have realized that biofuels based on corn (maize) will not expand as rapidly as had been hoped, given growing concerns about rising food prices and the suitability of corn as a feeder fuel.

Biofuels stocks are near to their lowest point in the past year, Wilder notes, even as the index hit a new high on growing investor confidence in wind and solar-power companies. Wilder notes that many of the solar companies, in particular, are starting to perform like regular industrial firms in a rapidly expanded market, rather than as exotic start-ups. ?The 'alternative' label is dropping off,? he says.

There's clearly been a flood of money into stocks in these areas but, Wilder thinks, opportunities remain in sub-sectors that have so far been less fashionable, such as geothermal power and companies that specialize in energy efficiency. In the stock markets, at least, ?efficiency hasn't been discovered yet?, he says.