"Put all your eggs in one basket -- and watch that basket," was steel magnate Andrew Carnegie's advice to the would-be wealthy. And perhaps no one illustrates Carnegie's wisdom better than Supreme Court Justice David H. Souter.

Long known as the most private and frugal member of the court, Souter, 64, recently became one of the richest, thanks to a decision he made years ago to invest his savings in his native New Hampshire -- and keep them there.

Our story begins in 1988, when Souter was an associate justice of the New Hampshire state Supreme Court. That year, a friend, Bruce Clow, launched a bank in Concord, the state capital. The board of directors also included Thomas D. Rath, Souter's close friend and successor as New Hampshire attorney general. Offered a chance to get in on the ground floor, Souter bought 32,000 shares at $5 a share, according to information reported by the Chicago Tribune at the time of Souter's 1990 Senate confirmation hearings.

It was a relatively risky move for Souter, who did not come from great family wealth and who had mostly worked in state government since graduating from Harvard Law School in 1966. When Souter reported total assets of $627,000 to the Senate Judiciary Committee in mid-1990, the $160,000 investment stood out as the largest item, just ahead of his family home in rural New Hampshire valued at $150,000.

But the bank, Horizon Bank & Trust, prospered and soon became a merger target as the region's financial sector entered a period of consolidation that continues today.

Each time one bank acquired another, shareholders in the target bank reaped large profits. Typically, they got the value of their stock, which had appreciated since they bought it, plus a "merger premium." Such premiums averaged about 33 percent during the last decade, according to Eric Hovde, an investment banker who specializes in financial institutions.

Souter went through this cycle repeatedly.

In 1995, Horizon was bought out by another New Hampshire bank, Peterborough Savings Bank, leaving Souter with a stake in the resulting Primary Bank worth between $500,000 and $1 million, according to his 1996 financial disclosure form.

Three years later, Granite State Bankshares, also of New Hampshire, swallowed up Primary. Souter's shares were now worth between $1 million and $5 million, according to his 1999 disclosure form.

And in 2003, Chittenden Corp., a Vermont-based firm, merged with Granite State, propelling the value of Souter's investment over the $5 million mark -- though exactly how far the justice is not required to say. The form indicates only that his stock is now worth between $5 million and $25 million.

"I highly doubt it's $25 million," Hovde said. "But I'd bet it's 6, 7 or 8 million." If so, Souter, who gets a $194,300 annual salary as an associate justice, would be at least 10 times richer than he was when President George H.W. Bush nominated him to the court 14 years ago.

Not a bad return, but hardly unusual for bank stocks in the New England region, according to Clow, who is now president of Laconia Savings Bank in Laconia, N.H.

"The trick was to have patience and keep it," Clow said.

Souter is now among the four wealthiest justices. Through a court spokeswoman, he declined to comment for this column.

This year, Ruth Bader Ginsburg reported assets worth between $5.6 million and $23.4 million; Stephen G. Breyer reported assets worth between $4 million and $15 million; and Sandra Day O'Connor reported assets worth between $2.9 million and $6 million.

But, while each of those justices has a well-to-do spouse whose holdings are included in the reported figures, Souter is single.

Souter's low-overhead lifestyle undoubtedly helped him build wealth. Never married, childless and famously unconcerned with the trappings of power, he lives in a rented apartment in the District and spends the court's summer recesses at his New Hampshire house. For years, his lunch menu has been the same: an apple and a cup of yogurt.

In the last two years, Souter plowed another large chunk of money into a New Hampshire startup, buying between $100,000 and $250,000 worth of stock in BizNews24.com of Bedford. Souter was one of the financial "angels" who helped get the streaming-video venture off the ground, according to Karen Dapkus, its vice president for administration.

"He loved the technology and he wanted to make some money," Dapkus said. "He's a New Hampshire kind of guy."

Supreme Court Justice David H. Souter's 1988 purchase of shares in a New Hampshire bank for $160,000 is now worth more than $5 million.