Securities regulators and some financial firms are making it more difficult for investors to pile on when stocks are falling and further drive down prices.

The Securities and Exchange Commission, facing years of criticism, has begun to crimp the ability of traders who bet against stocks to depress prices by selling millions of shares they don't possess, known as naked short selling. And some financial firms have cut back on lending to traders who want to bet against stocks.

The result: The number of stocks in which big chunks of shares haven't properly been delivered to investors has plummeted, to a daily average of 79 in the three months ending in March from 529 in the first nine months of 2008, according to an analysis of trading data from major stock exchanges.

At issue are short sellers, traders who sell borrowed shares, betting they can replace them later with shares bought at a lower price.

Critics say short sellers, with the aid of brokerage firms, cause these delivery failures by shorting stocks without first borrowing shares, as required by securities law. Such activity drives down stocks by adding to the selling pressure.