SÃO PAULO--Despite the collapse of his fortune and some of his companies, Brazilian businessman Eike Batista has returned to Brazil's main stock-market index, thanks to the modest resurgence of his mining company.

Mr. Batista's MMX Mineração e Metálicos SA joined the Ibovespa index Monday, and will remain there at least until Aug. 29, when the exchange operator next updates the index. MMX shares will account for 0.02% of the index.

BM&FBovespa SA changes the composition of the index three times a year, largely based on the volume of stock trading.

As MMX joins the Ibovespa, a company Mr. Batista used to control, Prumo Logística SA (formerly LLX Logística SA), dropped out of the index. Mr. Batista sold control of Prumo to U.S.-based EIG Group last year.

Mr. Batista's companies made him Brazil's richest man, and he vowed to become the richest person in the world. But his wealth plunged from more than $30 billion in 2012 to less than $1 billion after his oil company, Óleo e Gás Participações SA failed to meet production targets.

OGP's failure drove his other companies into turmoil, and most, including MMX, dropped out of the Ibovespa index after BM&FBovespa changed its methodology to remove penny stocks.

Last week, public prosecutors said they are investigating BM&FBovespa over whether the exchange held any blame for the collapse of Mr. Batista's companies. Federal police are investigating Mr. Batista and his once-high flying oil company, which is trying to exit from bankruptcy.

Meanwhile, for the first time in three decades, a bank will be the largest stock in the Ibovespa. Itaú Unibanco Holding SA will account for 9.973% of the index, overtaking the two companies that have dominated the index over this extended period: state oil company Petróleo Brasileiro SA and metals and mining firm Vale SA. Their shares fell to 7.795% and 5.640% of the Ibovespa, respectively.

Brazil's private sector bank shares have risen in value faster than the main index in recent months, as their conservative approach to credit portfolios generates less default and, as a result, less provisions charges.

At the same time, the government's intervention to control fuel prices at Petrobras has hurt the company's shares, while signs of a slowdown in China and the possible impact on iron ore demand have hit Vale's stock.

Write to Rogerio Jelmayer at rogerio.jelmayer@wsj.com

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