Citadel's Ken Griffin believes stock valuations are getting high, but there are still economic forces that can cause the bull market to continue awhile longer.

"I think we're in the seventh inning of this market rally," he said in an exclusive interview with CNBC's Leslie Picker on Monday. "Valuations are stretched. We're not in the sort of classic mania that you get at the very end of the bull market."

The investor, however, said the environment for stocks is still "constructive." He noted the economy's low inflation, low interest rates and decent sales growth from companies.

These "combined create a very strong underpinning for valuations," he added.

Griffin founded Citadel in 1990 and is its CEO. The hedge fund firm has more than $27 billion in assets under management. He has a net worth of $8.6 billion, according to Forbes.



