"Dear @BillGates there is an easy way to short bitcoin," Winklevoss said in a tweet Monday afternoon. "You can short #XBT, the @CBOE Bitcoin (USD) Futures contract, and put your money where your mouth is!"
A representative for Gates did not immediately respond to a CNBC request for comment.
Earlier on Monday, Gates said on CNBC's "Squawk Box" that "bitcoin and ICOs, I believe completely [they're some] of the crazier, speculative things."
"I agree I would short it if there was an easy way to do it," he said. Gates said someone once gave him some bitcoins for his birthday, but he sold it a few years later.