The physiological changes in Tetranychus urticae selected with spiromesifen+abamectin (S+A) mixture formulation were demonstrated by biochemical and molecular methods. The susceptible (GSS) population of T. urticae, was selected with S+A and made resistant with increasing doses of the mixture formulation. The resistant population was divided into two, and selection pressure was terminated in one population which is named IR (Interrupt resistant). In the other population, the selection with S+A was continued and the population named IR2. Activities of some detoxification enzymes (esterase, glutathione-S-transferase (GST) and cytochrome P450) were investigated in the GSS and IR populations. The activity of esterase, GST and P450 enzymes were increased by 1.77, 2.57 and 2.58-fold, respectively, in the IR population compared to the GSS population. Also, investigation of synergistic effects with esterase inhibitor (TTP), GST inhibitor (DEM) and P450 inhibitor (PBO) in this population revealed that all three synergists showed significant synergistic effects in IR population. In molecular studies, when the population was screened for the presence of two previously identified glutamate channel mutations (G314D and G326E) by qPCR with TaqMan probes no mutations could be detected in glutamate channels. and the CTD domain of acetyl coenzyme-A carboxylase (ACCase). Furthermore, possible genetic mutations in the biotin carboxylase domain (BCD) and carboxyl transferase domain (CTD) of the ACCase target sites were determined by sequencing. Although a single amino acid mutation G37D in the BCD of the ACCase gene of the IR and IR2 populations was detected, its association with spiromesifen resistance was not confirmed. The results indicated that increased detoxification and possible target site mutation may be responsible for the S+A resistance in the IR and IR2 populations.