Bacterial populations in Frankliniella occidentalis (Pergande) (Thysanoptera: Thripidae) collected in diverse California environments consisted of two bacterial symbionts: BFo-1 and BFo-2 (B = bacteria, Fo = Frankliniella occidentalisi numbers reflect different types). Dual infections of BFo-1 and BFo-2 were found in 50% of the thrips, 18% had neither bacterium, and 24 and 8% were infected solely with BFo-1 and BFo-2, respectively. No other bacteria consistently infected F. occidentalis. Dual infections occurred more often in male thrips and in thrips of both sexes from southern mountain and valley sites. As average collection year or month minimum temperature decreased, infections of BFo-1, alone or in dual infections, increased significantly. As yearly precipitation increased, infection with BFo-1 alone also increased. F. occidentalis color morphology did not affect bacterial infection. BFo-1 created weak biofilms at 25 and 32°C; BFo-2 made strong biofilms at 25°C and no biofilms at 32°C. When the bacteria were grown in culture together, weak biofilms formed at both temperatures studied, although there was no way to determine what each bacterium contributed to the biofilm. BFo-1 and BFo-2 grew at similar rates at 25 and 30°C. Our data show BFo-1 and BFo-2 occur in natural populations of F. occidentalis and support the hypothesis BFo have a symbiotic relationship with F. occidentalis. Regional differences in bacterial prevalence suggest bacterial infection is associated with environmental conditions, and altitude, temperature, and precipitation may be important factors.