The symbiosis of plants with arbuscular mycorrhizal fungi can improve crop growth by enhancing nutrient uptake and by increasing plant tolerance to certain pests. Pest populations also can be affected, but the effects of mycorrhizal colonization of plant hosts on species of thrips are unknown. We inoculated 2-week-old bell pepper seedling roots with a commercial mycorrhizal mixture of Glomus species (Glomales: Zygomycotina) and evaluated the effects 8 wks later on the bell pepper pest, western flower thrips (Frankliniella occidentalis (Pergande)). Females that were 10-days-old after adult emergence significantly preferred plants inoculated with arbuscular mycorrhizae compared with noninoculated plants in a laboratory choice experiment. Their numbers were greater on inoculated plants 48 h after release into cages containing inoculated and noninoculated plants. A no-choice laboratory experiment compared performance of F. occidentalis on noninoculated and inoculated bell pepper plants. Females that were 10-days-old after adult emergence were released into cages containing individual plants, and there were significantly greater total thrips (larvae, pupae, and adults) on treated plants. Further testing over a range of plant host fertility levels is needed to evaluate the potential for increased pest damage from F. occidentalis on crops growing in mutualistic symbiosis with arbuscular mycorrhizal fungi. Additional interactions with other microorganisms and natural enemies are possible under field conditions.