The spill was reported at about 10 a.m. at 1230 North McDowell Blvd., according to the Petaluma Fire Department.
It is unclear how the fuel spilled. Heavy rainfall moved some of it quickly down drains on Clegg Street headed to Highway 101, fire officials said.
FedEx staff and Petaluma fire crews helped contain the flow, although the cleanup effort continues and "will not be stopped until there is complete cleanup of the diesel fuel from the area," officials said.
Vacuum trucks and crews with absorbent pads will continue to mop up and dispose of the fuel throughout the night.
Crews from Oakland-based Morgan Environmental were also sent in for cleanup help, fire officials said.
