The company's shares dipped to $17.53 yesterday before closing at $17.58, down 3.72% on Monday's close, which gives it a market capitalisation of $24.36bn.
It hit a five-year low last week when it fell to $17.75.
Its current share price is almost half the $33 that Microsoft offered with its $44.6bn (£25.2bn) takeover bid in May. Yahoo! said at the time that it would only settle for $37 a share.
Yahoo's lowest point came in late September 2001, when it languished around $4.40, after peaking in the dotcom bubble at around $102 in January 2000.
Yahoo! executives are due to deliver an autumn update on the company's strategy tomorrow.
The announcement will come six weeks after shareholders voted in favour of Jerry Yang, Yahoo!'s chief executive, and the company's other executives, maintaining their positions on the Yahoo! board.
Their leadership was called into question after they failed to reach an agreement with Microsoft.
Separately, Yahoo! has hired Joanne Bradford, Microsoft's former top saleswoman for web advertising, as senior vice-president for US revenue and market development.
Bradford will oversee sales, market development for advertisers, small business and the Yahoo! HotJobs recruiting business.
She will report to Hilary Schneider, executive vice-president of Yahoo's U.S. business.