Never mind the decline in small-cap stocks—mid caps are performing even worse, Credit Suisse strategist Lori Calvasina said Thursday.

"We are very, very worried about the mid-cap space at the moment," she said. "We think it has the worst valuation story in the U.S. equity market at the moment. It looks just as bad as small caps did to start the year."

On CNBC's "Fast Money," Calvasina said that valuations were stretched for mid caps in the Russell Midcap Index.

While large-cap stocks are trading around 16 times earnings and are around 17 times, mid-cap stocks have a multiple of 18.

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"But what gets really interesting, is when you compare the three size segments versus each other," she said. "So, when we look at mid-caps right now they're basically pretty close to their May '07 highs in absolute terms. They're basically near records vs. large caps near all-time highs."

Calvasina also said that mid caps look as expensive as small caps since the tech bubble.

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"So you've got all these extremes on valuations that have been the same despite the nasty September that they had," she added.