Cross-resistance, resistance mechanisms, and mode of inheritance of spinosad resistance were studied in the weste flower thrip, Frankliniella occidentalis (Pergande).Spinosad (naturalyte insecticide) showed low cross-resistance to prothiophos (organophosphorus insecticide) and chlorphenapyr (respiratory inhibitor) showed some cross-resistance to thiocyclam (nereistoxin). The synergists PBO (piperonyl butoxide),DEM (diethyl maleate), and DEF (s,s,s-tributyl phosphorotrithioate) did not show any synergism on the toxicity of spinosad in the resistant strain (ICS), indicating that metabolicmediated detoxification was not responsible for the spinosad resistance, suggesting that spinosad may reduce sensitivity of the target site: the nicotinic acetylcholine receptor and GABA receptor. Following reciprocal crosses, dose-response lines and dominance ratios indicated that spinosad resistance was incompletely dominant and there were no mateal effects. The results of backcross showed that spinosad resistance did not fit a single-gene hypothesis, suggesting that resistance was influenced by several genes.