Marie Henriques found a "dark furry object" in a Topic bar which had been bought from a kiosk in a central London underground station, Horseferry Road Magistrates Court was told.
Sweet manufacturers Mars UK Ltd deny selling food so contaminated it could not be expected to be used for human consumption. The prosecution has been brought by Westminster Council which identified the object as a mouse.
The court was told the chocolate bar was bought for Miss Henriques, 31, at Piccadilly Circus in April last year. Timothy Spencer, for Westminster Council, said: "When she put the last piece into her mouth she bit on something hard. She saw a black, furry object surrounded by caramel and nuts. She thought it was a peach stone. She showed it to a colleague who reported seeing thick black fur, and red stuff. She said it was a mouse."
Miss Henriques, from Leyton, east London, immediately contacted Mars, which asked for a sample. She retained half of the object, which was sent to the public analyst, who identified it as the head and shoulders of a rodent. It was later identified by the Natural History Museum as being part of a mouse.
Magistrate Tim Workman heard that the chocolate bar had been manufactured by Mars at Slough in February 1997. Hazelnuts were sent to a next- door warehouse company, Fullers, which sent them by conveyer belt to Mars.
Mr Spencer said that Mars had been told in January the warehouse had an infestation of mice. In January and February more than 80 mice were found by pest control officers.
But, said Mr Spencer, Mars did not insist on Fullers getting its staff to conduct visual inspections to check that the batches of hazelnuts were uncontaminated.
The case continues.