Warren Buffett's Berkshire Hathaway has been welcomed into the benchmark S&P 500 index with a surge of trading just before today's close at the New York Stock Exchange.

CNBC's Maria Bartiromo reports there was an enormous crowd around the Berkshire post on the floor of the NYSE.

Index mutual funds that mimic the performance of the S&P had to buy Berkshire Class B shares because the stock is being added to the index after today's close.

Today's Wall Street Journal notes that over $1 trillion of investors' money "directly tracks" the S&P.

Berkshire B shares fell to a low of $74.57 around 1p ET today, and then rebounded to end the day at $76.90, a gain of $0.21, or 0.27 percent.  That's 13.1 percent above the $68 close on January 26, the day S&P announced Buffett would join the S&P.

Some of the buying of B shares may have been the result of investors moving out of the much higher priced A shares.  They closed down $950 at $114,000 on enormous (for the stock) volume of over 18,000 shares.  CNBC producer Robert Hum at the NYSE reports there was a huge imbalance of sell orders that kept trading open until almost one hour after the closing bell.

In one especially large trade at 4:41p ET, just over 8-thousand shares went for $114,000 each.

Berkshire is replacing Burlington Northern in the S&P, now that the railroad is officially a Berkshire subsidiary following today's closing of Buffett's $26 billion "bet on America."

A 50-for-1 stock split of the B shares, prompted by the Burlington deal, cleared the way for Buffett & Co. to join the elite S&P 500.  Before the split, the stock's four-digit price limited liquidity. 

Current Berkshire stock prices:

Class B:

Class A:

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