With an estimated net worth of $40 billion (down $18 billion over the past 12 months), Microsoft ( MSFT) chairman Bill Gates is now the world's richest man, according to Forbes' latest ranking of the world's billionaires. (Last year's top billionaire, Warren Buffett fell to second place, with an estimated net worth of $37 billion.)

So where does Gates put his money to work in the stock market?

Here are a few of Gates' largest holdings via Cascade Investments, his private investment vehicle.

  • Canadian National Railway (CNI)
  • Republic Services (RSG)
  • Fomento Economico Mexicano (FMX)
  • Berkshire Hathaway (BRK.A)
  • For more, check out the Cascade Investment portfolio on Stockpickr.com.

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