Police Sgt. Lyn Tomioka said last week that in the past month, at least four toilets had been set on fire in the Russian Hill area.
Since then, at least one more such fire occurred, in the 1500 block of Vallejo Street early Saturday morning, police said.
The notice sent out Tuesday asks workers to lock the toilets at the end of the workday, keep them away from buildings and trees, and to report any suspicious activity to the Police Department at (415) 553-0123.
Fire Lt. Mindy Talmadge did not have details about how the fires were set.
The fires have not injured anyone and remain under investigation.