The trader thought to have cost one of France's biggest banks 4.9bn euros ($7.1bn; £3.7bn) is in police custody.
Jerome Kerviel, the reported rogue trader
Jerome Kerviel is being held for questioning about the alleged fraud at the French bank, Societe Generale.
The 31-year-old trader had not been seen since the financial scandal broke on Thursday. His family and lawyer have insisted he is innocent.
On Friday police searched Mr Kerviel's flat in an upmarket Paris district taking away a number of briefcases.
Police also visited the headquarters of Societe Generale, where they were given documents and computer disks relating to the alleged fraud.
Mr Kerviel arrived at the headquarters of the financial police on Saturday at 1400 (1300 GMT) for questioning.
Correspondents say the police never launched a manhunt for the trader and it is likely they knew all along how to find him.
Mr Kerviel may be held for 24 hours without charge, with a possible extension if granted by a judge.
French prosecutors are conducting a preliminary investigation based on a complaint from the bank, and on two complaints from small shareholders in the bank, reported Associated Press.
Mr Kerviel was responsible for betting on the markets' future performance.
Though Societe Generale has yet to officially name Mr Kerviel, it has filed a legal complaint against the trader, accusing him of defrauding the bank by making unauthorised financial trades.
The bank says it discovered the fraud last weekend and began to offload the trader's losing bets on Monday - when world markets fell heavily.
In an interview with French newspaper Le Figaro, bank chairman Daniel Bouton called allegations that the bank had triggered the falls "absurd".
He defended the bank's handling of the scandal, and insisted it would withstand the losses.
As the recriminations continue, both Societe Generale shareholders, analysts and the French government have questioned how the rogue trader was able to operate alone.
SOCIETE GENERALE IN FIGURES
Founded in 1864
467bn euros in assets under management (as of June 2007)
22.5m customers worldwide
120,000 employees in 77 countries
"It is difficult... to imagine how one person alone could, in a relatively short period of time, cause such considerable losses," said French Prime Minister Francois Fillon.
"They are saying all of this was cunningly concealed, but somebody must have been funding the collateral or whatever was needed to sustain those positions," said Derek Chambers at Standard & Poor's Equity Research.
In an additional twist, the bank has said Mr Kerviel might not have personally benefited from his alleged fraudulent transactions.
The French government has expressed anger that Societe Generale did not inform it immediately after the losses were discovered.
French President Nicolas Sarkozy called the events at Societe Generale a "large-scale internal fraud", but added that the losses "do not affect the solidity and reliability of the French system".
Societe Generale said the fraud was based on simple transactions, but concealed by "sophisticated and varied techniques".