The bicyclist was riding downhill on westbound Fountaingrove Parkway when a blue extended-cab pickup that was traveling east turned left from a turn pocket onto Lake Thomas Harris Drive, Sgt. Rich Celli said.
The bicycle was in the right lane when it struck the right passenger door of the pickup. There is no bicycle lane in that area of westbound Fountaingrove Parkway, Celli said.
The driver of the truck, 22-year-old Adam Bigham of Sebastopol, told police he did not see the bicyclist. Crash investigators will examine road conditions to determine if the sun was in Bigham's eyes at the time of the 7 a.m. crash.
That area of the road, a major east-west route in Santa Rosa with a 40 mph speed limit, was in shade immediately after the collision, Celli said.
Bigham stopped at the collision scene and is cooperating with the investigation. Alcohol or drug use by the driver or the bicyclist are not considered factors in the crash, Celli said.
There have been four other collisions that killed bicyclists in Sonoma County since May.
"There's no pattern," Celli said. "Sometimes the bicyclist is at fault, sometimes it's the vehicles. Sometimes it's inattention by both."
The bicyclist's name has not been released because his next of kin have not been notified, the Sonoma County coroner's office said.
Celli said the bicyclist was wearing spandex and appears to have been a recreational rider.