The limo caught fire at around 10 p.m. Saturday on one of the busiest bridges on San Francisco Bay, California Highway Patrol officer Art Montiel told The Associated Press.
Five of the women were trapped, but the four other women managed to get out after the vehicle came to a stop on the San Mateo-Hayward Bridge, the patrol said.
Rosita Guardiano told the San Francisco Chronicle that the woman for whom the bachelorette party was being thrown was to be married next month. Guardiano said her daughter was one of the survivors.
Investigators haven't determined what sparked the fire, but the patrol said the white stretch limo became engulfed in flames after smoke started coming out of the rear of the vehicle.
A photo taken by a witness and broadcast on KTVU-TV showed flames shooting from the back of the 1999 Lincoln Town Car.
Aerial video shot after the incident showed about one-third of the back half of the limousine had been scorched by the fire. Its taillights and bumper were gone and it appeared to be resting on its rims, but the remainder of the vehicle didn't appear to be damaged.
The driver of the limo - 46-year-old Orville Brown of San Jose - was the only person to escape unhurt.
It wasn't clear how he managed to escape without injury. Investigators Sunday afternoon were still seeking witnesses, the CHP said.
"Four people got out, as far as what was going on inside, I don't know," CHP officer Jeremy Lofstrom said Sunday. CHP investigators Sunday afternoon were still seeking witnesses to the incident.
All five women were pronounced dead at the scene. Autopsies were being conducted, San Mateo County Supervising Deputy Coroner Michelle Rippy said.
The company that operated the limo was identified as Limo Stop, which offers service through limousines, vans and SUVS.
A telephone message left at the company seeking comment by The Associated Press wasn't immediately returned. Attempts to reach the driver were also unsuccessful.
Guardiano said her daughter - 42-year-old Mary Grace Guardiano of Alameda - was being treated for smoke inhalation.
The three other women who escaped the fire, Jasmine Desguia, 34, of San Jose; Nelia Arrellano, 36, of Oakland; and Amalia Loyola, 48, of San Leandro, were taken to hospitals to be treated for smoke inhalation and burns, the patrol said.
Desguia and Loyola were listed in critical condition, said Joy Alexiou, a spokeswoman for Valley Medical Center. The condition of Arrellano, who was taken to another hospital, was not known.