David Rubenstein was the latest big-name investor to express some level of concern about market valuations, though he remains generally optimistic about prospects for the U.S.
Speaking Thursday at the SkyBridge Capital SALT 2014 hedge fund conference, the billionaire head of the Carlyle Group said financial markets no longer look as cheap as they did not so long ago.
"The markets are not cheap," Rubenstein said. "In the buyout world right now it's actually difficult to find deals. Companies that are publicly traded are at their highs, and they have a lot of cash."
His comments come a day after fellow billionaire hedge fund magnate David Tepper advised a more cautious approach to the market, telling CNBC that he has reduced his equity exposure from full investment to 60 percent. Stocks were in aggressive selloff mode Thursday, with the major indexes each giving up more than 1 percent.
Addressing the deal climate specifically, Rubenstein said "there's a bit of frothiness in it." U.S. merger and acquisition volume is up 35 percent in dollar volume to $506 billion, according to Dealogic. Total deals activity, though, is down 7 percent.
More broadly, though, he still considers the U.S. the best place globally to invest, followed by China.
"You're going to do quite well investing in China if you know what you're doing," Rubenstein said. "The potential for selling product in that market is staggering."
—By CNBC's Jeff Cox.