Prey stage preference, switching and mutual interference of Phytoseius plumifer (Acari: Phytoseiidae), a predator of the two-spotted spider mite (TSSM), Tetranychus urticae were evaluated in a growth chamber at a temperature of 27±2°C, relative humidity of 50±5% and a photoperiod of 16:8h (L:D) on the leaf disc of soybean (Glysine max (L.)). The results of no-choice experiments revealed that P. plumifer consumed egg, larva, protonymph and male stages of TSSM more than deutonymph and female but in choice experiments, the predator significantly preferred immature stages of its prey. Switching behavior of the predator studied on the larval and deutonymphal stages of TSSM at 30:70, 40:60, 50:50, 60:40 and 70:30 ratios (larva:deutonymph). The predator switched from one stage when it becomes rare to another more abundant stage. Furthermore, a significant relationship between the logarithm of the predator density and the logarithm of per capita searching efficiency of P. plumifer was recognized with the interference coefficient (m) of -0.399. This revealed that as the predator density increased, the per capita predation rate and per capita searching efficiency decreased significantly. Although P. plumifer is not a specialized predator of the TSSM, it potentially aids in enhancing biological control of this mite.