The arson occurred at about 5:25 p.m. in the 700 block of Chestnut Street, police spokeswoman Sgt. Lyn Tomioka said.
Most of the arsons have occurred late at night or early in the morning, but Sunday's was the second in the past three weeks that took place during the daytime, according to Tomioka.
The other happened Jan. 22 in the Marina District. It was reported just after 1 p.m. near the intersection of Marina Boulevard and Scott Street, Tomioka said.
No arrests have been made in the string of lavatory arsons, most of which have taken place in the Russian Hill area.
