Despite California's 1994 statewide smoking ban, exposure to secondhand smoke (SHS) continues in California's Indian casinos. Few data are available on exposure to airborne fine particles (PM2.5) in casinos, especially on a statewide basis. We sought to measure PM2.5 concentrations in Indian casinos widely distributed across California, exploring differences due to casino size, separation of smoking and non-smoking areas, and area smoker density. A selection of 36 out of the 58 Indian casinos throughout California were each visited for 1–3 h on weekend or holiday evenings, using two or more concealed monitors to measure PM2.5 concentrations every 10 s. For each casino, the physical dimensions and the number of patrons and smokers were estimated. As a preliminary assessment of representativeness, we also measured eight casinos in Reno, NV. The average PM2.5 concentration for the smoking slot machine areas (63 μg/m3) was nine times as high as outdoors (7 μg/m3), whereas casino non-smoking restaurants (29 μg/m3) were four times as high. Levels in non-smoking slot machine areas varied: complete physical separation reduced concentrations almost to outdoor levels, but two other separation types had mean levels that were 13 and 29 μg/m3, respectively, higher than outdoors. Elevated PM2.5 concentrations in casinos can be attributed primarily to SHS. Average PM2.5 concentrations during 0.5–1 h visits to smoking areas exceeded 35 μg/m3 for 90% of the casino visits.