TORONTO -- Police in Toronto say they're trying to identify a man who allegedly sexually assaulted a woman on a GO Transit bus.

They say a 19-year-old woman was on a GO bus that left Yorkdale station for Durham Region on Monday evening.

They say a man sat beside the woman and began a conversation, which allegedly became sexual in nature.

Investigators allege that the woman was sexually assaulted somewhere between Yorkdale station and Ajax, Ont.

The man is described as 25 to 30 years old, 5-foot-9 to 6 feet tall, with a medium build, and shoulder-length brown hair parted down the middle.