Twitter is joining the S&P 500, a benchmark index of the U.S. stock market, marking another milestone for the social media company.

  

The San Francisco-based company will replace Monsanto Co. in the index. Monsanto, an agricultural products company, is leaving the S&P 500 because it is being acquired by Germany's Bayer AG.

  

S&P Dow Jones Indices says the change will become effective prior to the open of trading on Thursday.

  

Netflix will replace Monsanto on the S&P 100, a subset of the S&P 500 for some of the largest companies.

  

The announcement of the changes came after the close of regular trading hours. Twitter's stock rose 3.5 percent in after-hours trading.