Casey’s General Stores Inc., headquartered in Ankeny, has been added to the S&P 500, a stock market index that tracks the performance of 500 leading large-cap U.S. companies.

This is the first time Casey’s has been included on the S&P 500 since becoming a publicly traded company in 1983. The convenience store chain is the second Iowa-based company to be included on the index. Principal Financial Group, based in Des Moines, has been on the S&P 500 since 2001.

“Being added to the S&P 500 marks a significant milestone that reflects the enduring strength of our business,” Darren Rebelez, Casey’s president and CEO, said in a news release. “We remain committed to disciplined growth, delivering exceptional experiences to our guests and creating long-term value for our shareholders.”

The S&P 500 is the primary indicator used by investors to measure the health of the U.S. stock market.

Companies on the S&P 500 are selected by a group of experts at S&P Dow Jones Indices, according to an article by Charles Schwab, a multinational financial services company. Selections of companies to be added or removed occur four times a year.

To join the S&P 500, a company must have been profitable for the last four quarters and in the most recent quarter specifically with a market capitalization of at least $22.7 billion, according to the article. The company must also meet float and volume criteria. Companies must be based in the United States and traded on one of several approved U.S. stock exchanges.

Casey’s, a Fortune 500 company, is traded on the Nasdaq Composite Index. The company has been consistently profitable in the past four quarters with net income and earnings per share growing due to strong inside store sales and increased fuel margins.

The company has a market capitalization of $28.58 billion, making it the world’s 842nd most valuable company by market cap, according to CompaniesMarketCap.com. Market capitalization is the value of a publicly traded company and can be determined by multiplying the share price with the amount of outstanding shares.

Casey’s is the third largest convenience store chain in the U.S., operating over 2,900 stores in 19 states.