Nigeria's President, Goodluck Jonathan, has sacked the managing director and several other executives of the state oil company, NNPC.
A statement said the action had been taken in the interests of "greater transparency and accountability".
Nigeria is one of the world's biggest oil producers, but the sector is widely seen as being plagued by corruption.
A recent report by legislators found $6.8bn (£4bn) had been lost to fraud in the past two years.
The fuel sector probe was set up in the wake of angry nationwide protests in January after the government tried to remove a fuel subsidy.
Oil accounts for some 80% of Nigeria's state revenues but it has hardly any capacity to refine crude oil into fuel, which has to be imported.
The government subsidises the cost of fuel and had said this cost the country $8bn in 2011.
The 205-page parliamentary report revealed alleged wrongdoings involving oil retailers, the state Nigeria National Petroleum Corporation (NNPC) and Nigeria's Oil Management Company.
A key finding was that while the government had been paying the subsidy for 59m litres of fuel a day, Nigeria only consumes about 35m litres.
"To further strengthen the on-going reforms... President Jonathan has approved the re-composition of the executive management team of the NNPC," Reuters news agency quotes the statement from the president's office as saying.
NNPC Director Austen Oniwon is being replaced by Andrew Yakubu, a chemical engineer who has held other positions within the oil industry.
In February, Nigeria's former corruption chief Nuhu Ribadu was appointed to head a new task force to police the oil sector by tracking revenues paid to the government, and monitor crude oil production and exports.
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