At about 8:15 a.m. Tuesday, a school bus driver reported seeing a monkey cross the road on Captain Nurse Circle, humane society spokeswoman Carrie Harrington said.
The bus driver's report was "very vague," and the only description was that the animal had a tail about 18 inches long, Harrington said.
The humane society then received a call at about 5 p.m. Tuesday from a woman who said her daughter had spotted a monkey while driving in the same area on Thursday, Harrington said.
The woman said that while they were driving, her 11-year-old daughter said, "Mom, look, there's a monkey over there," but the woman did not see the animal herself, Harrington said.
After seeing news reports about the bus driver's sighting, the woman asked her daughter about the incident again and the girl insisted she saw the monkey, Harrington said.
The girl described the animal as being dark brown in color and about the size of a 2-liter bottle of soda, she said.
The Marin Humane Society will have an officer patrol the area, and is asking anyone who sees the animal to call the humane society at (415) 883-4621.
Harrington said she cannot recall any previous monkey sightings in Marin County.
"This is definitely a first for us," she said.