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San Francisco · CA
When does flea season start in San Francisco?
San Francisco doesn't get a real flea off-season — every month stays warm enough for fleas to keep breeding, heaviest May–Oct.
Ticks in San Francisco
Local tick activity typically runs year-round, heaviest Feb–Nov. The species to know in California: western blacklegged tick, American dog tick, brown dog tick.
More on species and timing: California tick season.
Numbers come from NOAA U.S. Climate Normals (1991–2020) recorded at SAN FRANCISCO INTL AP — a climate-based estimate; real years wobble.
Frequently asked questions
When does flea season start in San Francisco?
It never fully stops — San Francisco's monthly averages support flea activity essentially all year, with the heaviest stretch May–Oct.
Is flea season in San Francisco different from the rest of California?
Not meaningfully — San Francisco tracks the California statewide estimate closely. Both are climate-based estimates, so treat the edges as fuzzy, not exact.
Do dogs in San Francisco need prevention outside flea season?
Yes — vets recommend year-round parasite prevention regardless of the local calendar. Indoor fleas ignore the weather entirely, and ticks in California are typically active year-round, a longer window than fleas get.