Shares in US mortgage finance agencies Freddie Mac and Fannie Mae are at their lowest levels in nearly two decades.
Their fall has revived speculation that the federal government will be forced to bail out the pair.
Shares in Fannie Mae, the biggest provider of housing finance in the United States, fell 27% to $US4.40 - their lowest since 1988.
Shares in Freddie Mac crashed by nearly 22% to $US3.25 - their lowest since 1990.
However, the Treasury has said that it has no plans bail out the two agencies, which underpin the US mortgage market.
They are government-sponsored enterprises and own or back almost half all US mortgages.
The Treasury gained the authority to bail out the Freddie Mac and Fannie Mae, including buying shares in the two companies if needed, in a rescue plan approved at the end of July.
On Monday a report by US financial weekly Barron's suggested that the chances of a government rescue were increasing.
The paper said a government bail-out would likely wipe out existing holders of the firms' shares.
