Giusti, 48, is charged with one felony count of violating a federal law that makes it a crime to make obscene, harassing or threatening interstate phone calls.
Dressed in dark blue jail garb and appearing slightly disheveled, a heavyset and bearded Giusti entered his plea before U.S. Magistrate Bernard Zimmerman in San Francisco.
According to an affidavit unsealed in federal court on April 8, Giusti made 48 threatening and sometimes obscene phone calls to the homes and offices of Pelosi and her husband over the two months before his arrest on April 7.
The affidavit states that Pelosi told investigators the caller had used crude and vulgar language to say that if she helped pass the health care bill she wouldn't have a home to return to in San Francisco.
The maximum penalty if Giusti is convicted would be two years in prison, one year of supervised release and a fine of $250,000.
Giusti has a previous criminal history that includes convictions in San Mateo County for threatening a Caltrain conductor and committing vandalism against BART. He has San Francisco convictions for perjury in a welfare fraud case and petty theft.
Giusti was also the target of a lawsuit filed by the Hamilton Square Baptist Church in September 2009 that alleged a "campaign of harassment" against the church.
Guisti will return to court Thursday. In the meantime, he is being held without bail.