NEW YORK — For big banks like Citigroup, the first quarter of 2009 may turn out to be the best of the year.

Citigroup Inc., JPMorgan Chase & Co., Goldman Sachs Group Inc. and Wells Fargo & Co. all impressed investors over the past week with earnings reports that were better than Wall Street analysts anticipated.

Citigroup posted a first-quarter loss to common shareholders of $966 million, or 18 cents per share, narrower than the 34 cent forecast of analysts surveyed by Thomson Reuters. But before paying dividends to preferred stockholders tied to a private stock offering in January 2008, the bank earned $1.6 billion, while revenue doubled from a year ago to $24.8 billion.

The banks' results showed Wall Street that its five-week-old rally was not in vain. Six weeks ago, Citigroup's chief executive Vikram Pandit and other bank CEOs ignited the rally by telling investors that January and February were profitable.

But the recent batch of reports did not change the widespread opinion that this year is going to be one of the toughest ever for the banking industry.

"There's life in this sector," said Gary Townsend, chief executive officer of Hill-Townsend Capital LLC, but also "plenty of questions related to the sustainability of the results."

So after bank stocks' strong bounce in March and April, "they're probably due for a breather," said Fox Pitt Kelton banking analyst David Trone.

Loan losses are only going to get worse as unemployment rises.

Citigroup saw some moderation in 30-day delinquencies in cards and mortgages, but its chief financial officer Ned Kelly warned: "One swallow does not make a spring."

When it comes to loan losses, "the elephant hasn't made its way through the python," Kelly said.

And the two biggest drivers offsetting loan losses at these banks were strong bond trading results and low borrowing rates. Borrowing rates are likely to stay low for a while, but they can't get much lower. And bank executives are not confident about keeping up trading revenues.

The first quarter saw a surge in corporate bond issuance as the credit markets started thawing from their frozen fourth quarter. JPMorgan Chase CEO Jamie Dimon said that it wasn't reasonable to expect record results to continue at its investment bank.

Citigroup's chief financial officer said the first quarter is historically the strongest for investment banking, and Goldman's chief financial officer, David Viniar, said the market is "still a dangerous environment."

Investors will get a better sense of how sick banks are in May, after the government finishes its "stress tests" designed to determined whether the companies would need more federal bailouts under various economic scenarios.

Citigroup said yesterday it is delaying the government's exchange of billions of dollars worth of preferred shares into common shares until the government completes its test.