South Africa's Murray and Roberts (M&R) said on Wednesday that Germany's ATM Holding GmbH has acquired a 25% shareholding in the engineering and construction group.
M&R said it will now engage with ATON regarding its intentions.
"We don't know what their intentions are at this stage - all we know is that they own 25% of the company," M&R head of investment Ed Jardim told Reuters.
ATON had previously acquired 4.49% share in M&R in 2015 and now holds about 25.5% of M&R's total ordinary share capital.
Coronation Fund Managers sold its 15% stake last week. Shares in the construction firm jumped to a near six-month high, although the company said in a statement that it was not aware of what had caused the rise in its stock.
Major shareholders Allan Gray and Sanlam Investment Management had also reduced their interest in the construction company on Tuesday.
ATON had previously held discussions between September 2015 and April 2016 with M&R regarding a potential merger of its mining contracting subsidiary, J.S. Redpath Holding, with M&R's underground mining business but an agreement could not be reached on the the deal's structure and value.
Shares in M&R were up 2% at 1531 rand ($117.23) by 0830 GMT. Shares in rival South African construction company Group Five fell 2.9% after it said its chief executive would leave as it reported its first six-month loss in 11 years.