MTN shares dropped more than 5 percent on Monday after an unconfirmed report in Nigeria said the mobile phone firm's Nigerian unit could face a fine of as much as $5.2 billion, traders said.
MTN declined to comment on the matter.
"The report is definitely the reason why it is moving. We just don't know how credible it is," said Petri Redelinghuys, a trader at Inkhunzi Investments in Johannesburg.
A second trader confirmed the share was falling as a result of the report.
Chris Maroleng, the Johannesburg-based spokesman for Africa's largest mobile operator, declined to comment.
The unconfirmed online report by Nigeria's Leadership newspaper said MTN was fined by the Nigerian Communications Commission for failing to disconnect subscribers with unregistered and incomplete SIM cards.
MTN was trading 5.32 percent at 180.80 rand by 1109 GMT.
- Reuters
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