The California Highway Patrol said the cyclist was hit as he was riding along Avenue 12 near Road 25.
A big rig hauling a trailer full of milk was driving up behind the cyclist when the cyclist swerved for unknown reasons and rode directly into the path of the big rig.
The driver of the truck swerved to try to avoid him, but the cyclist was hit by the trailer and died at the scene.
There's no word yet on the victim's identity. Authorities said he was in his thirties.