It happened around 12:00 Friday afternoon on the 500 block of Twin Oak Drive in Haverford Township.
The incident was reported by two 13-year-old girls.
One of the girls told police that as she walked pass a car, she saw a man with his pants down.
The man was allegedly holding a phone in one hand, while handling himself in an indecent manner.
The girls told a postal worker nearby and as the worker approached the vehicle, the driver turned quickly and fled.
The man is described as a white male in his early 20s, with dark hair and stubble facial hair. He is also described as being so tall that his head appeared to touch the inside roof of his car.
The vehicle is described as a older model compact car, possibly a Honda Civic. It was dark green and had a green and yellow checkered infant seat in the back.
Anyone with information is asked to contact the Haverford Township Police Department 610-853-1298 ext.1230.