What’s striking is that the Internet and biotech groups were the beating heart of irrational exuberance in that earlier bubble and they are setting off alarms again.

In the last few weeks, publicly traded stocks in these two sectors have begun falling, after a gravity-defying rise. The Nasdaq Internet index is down more than 11 percent since peaking on March 6. It gained more than 63 percent in the previous 12 months. The Nasdaq Biotechnology index peaked on Feb. 25. In the previous 12 months, it gained 90 percent. Since then, it has dropped 16 percent.

New I.P.O.s, meanwhile — along with companies being valued in mergers and acquisitions and other transactions — are commanding prices that some strategists say will turn out to be unsupportable.

“There are definitely speculative excesses in the market right now,” said Doug Kass, president of Seabreeze Partners Management. “I don’t think the whole market is in a bubble. But in biotech and some of the Internet stocks, there’s no question — we’ve certainly got bubblelike symptoms. And the I.P.O. market looks like a bubble, and that’s serious, because that’s where the first signs of the bear market that started in 2000 began.”

It’s not clear where these sectors are headed now. So far at least, they seem to be operating quite separately from the rest of the market. Figures from the Bespoke Investment Group tell the story. One important metric is the price-to-earnings ratio. In 2000, for the 10 biggest stocks in the S.&P. 500 by market cap, the P/E ratio was 62.6. Today, the comparable number is only 16.1. Back in March 2000, Cisco had the highest P/E ratio among the 10 biggest stocks, at 196.2, followed by Oracle, at 148.4. Those numbers were so high that when sentiment turned, the stocks plummeted.

Today, only one stock in the big 10 has a P/E above 30: Google, the sole Internet company in the group. Its P/E is 33.3, double the current average for the S.&P. 500’s 10 biggest companies, but compared with the levels that prevailed in 2000, it is reasonably priced. If earnings grow rapidly, Google could conceivably be profitable for investors at its current valuation.

The point is that even if prices are high in the overall market, they are being backed up by earnings to a much larger extent than in 2000. That’s important, because back then, when the dot-com bubble burst, the downdraft brought most companies down with it. And that’s why some people applauded when shares of King Digital, the Candy Crush maker, dropped 16 percent on their first day, while the rest of the market was largely unaffected.

Declines like that might be good news if the result is a less bubbly market over all, said Jeffrey Kleintop, chief market strategist at LPL Financial. “It’s O.K. if the market turns a little against some of the highfliers,” he said. “We seem to be seeing investors beginning to focus more on valuation this year, and that’s good because for many companies earnings will be growing.” And if the market cycle heads in a happy direction, earnings, not manias, will be driving the story.