Bill Gates owns $25 billion in blue-chip S&P 500 stock

Bill Gates built one of the country's most closely watched investment portfolios through Cascade Investment LLC, the firm that manages his personal fortune and helps fund the Gates Foundation.

Most people picture Cascade Investment holding familiar names such as Deere and Ecolab. Those are both in the mix.

But one company sits far above everything else in the portfolio, and it is not a technology stock.

It is a waste-and-recycling company that most Americans interact with every week.

Why Bill Gates' stock picks carry weight

Cascade Investment does not trade like a typical hedge fund. It tends to hold large, concentrated positions for years and rarely offloads these stakes in a hurry.

Cascade Investment also helps fund the Gates Foundation, one of the world's largest philanthropic organizations.

Growth inside Cascade's holdings helps support the foundation's global health and poverty work over time.

So when a single stock makes up more than half of Cascade's entire portfolio, it tells you something about how much conviction is behind that bet.

How big is Bill Gates' RSG stock stake ?

Republic Services is the largest holding in Cascade's portfolio. Cascade Investment holds 112.8 million shares of Republic Services (RSG) stock, worth roughly $25.16 billion, according to portfolio data TheStreet reviewed from Tikr .

That single position accounts for 53.75% of Cascade's entire portfolio, making it the firm's largest holding by a wide margin.

For comparison, Cascade's second-largest position, Deere & Co, is worth about $9.2 billion and accounts for 19.66% of the portfolio.

Ecolab, the third-largest holding, is valued at about $7.76 billion, or 16.58% of the portfolio.

The Republic Services stake also equals 36.84% of all RSG shares outstanding. In plain terms, Cascade Investment owns more than a third of the entire company.

Republic Services is one of the biggest environmental services companies in the United States and Canada. The company, incorporated in 1996, is based in Phoenix, Arizona.

Its business covers collecting and processing recyclable, solid, and industrial waste. It also handles transportation and disposal of hazardous and nonhazardous waste, along with landfill services and recycling centers.

Republic Services works with small containers, large containers, and residential customers, supplying containers, compactors, and curbside pickup, the company says.