The listing of MTN Nigeria, a unit of South African telecoms firm MTN Group, made it the second-largest firm on the Nigerian Stock Exchange, and since then its share price has risen from 90 naira to 140 naira.
A spokesman for Nigeria's financial crimes agency was not immediately available for comment.
"We received all regulatory approvals required to list our shares," MTN Nigeria said in a statement. "We are co-operating fully with the authorities."
(Reporting by Paul Carsten; Editing by Angus MacSwan and Alexander Smith)