American investors are returning from summer vacation to the cheapest stock market in almost 12 years, and some of the biggest fund managers say they’re ready to load up on shares of technology, energy, and industrial companies. Software makers in the Standard & Poor’s 500 Index last week were valued at an average 20.8 times estimated profit, the lowest since at least 1995, according to data compiled by Bloomberg. Industrial companies traded at 18.4 times earnings, lower than their average of 23.4 this decade. Oilfield-services provider BJ Services Co. last month was the cheapest in almost six years.

While the benchmark for American equity tumbled 9.4% between July 19 and August 15 on concern the worst housing slump in 16 years would slow economic growth, the index gained in August for the first time since May. President Bush and the chairman of the Federal Reserve, Ben Bernanke, reassured investors last week that they would prevent losses in credit markets from ending the six year expansion.

“The market’s probably seen the worst of it,” the Denver-based senior investment officer for AIM Investments, which oversees $160 billion, Fritz Meyer, said. “The Fed ultimately will ride to the rescue.”

Stocks are “on sale” and managers are “finding opportunity everywhere,” Mr. Meyer said. The S&P 500’s average price-to-earnings ratio of 16.8 for August was the lowest since November 1995, according to monthly data compiled by Bloomberg.

The slump, the steepest since March 2003, wiped out $1.41 trillion in market value. The S&P 500 rebounded 4.8% after the Fed on August 17 unexpectedly lowered the interest it charges banks and the index ended the month 1.3% higher.

Investors returning from vacation yesterday snapped up technology and energy shares, spurring a rally in American stocks on speculation prices haven’t caught up with earnings growth. The S&P 500 added 1.1% to 1,489.42.

Mr. Bernanke, speaking August 31 at the Kansas City Fed’s annual symposium in Jackson Hole, Wyo., said central bankers will “act as needed” to buoy the economy hampered by the abrupt rise in the cost of credit resulting from defaults in subprime mortgages. Mr. Bush said the same day that he will let the Federal Housing Administration, which insures mortgages for low- and middle-income borrowers, guarantee loans for delinquent borrowers, allowing them to avoid foreclosure and refinance at more favorable rates.