In this study, population density and yield loss caused by two-spotted spider mite (TSSM) on eight lines of pinto bean were studied under field conditions during two successive years (2009-2010). The experiment was arranged in completely randomized block design with subdivided plots and four replications in Lordegan fields, Chaharmahal va Bakhtiari province, Iran. The main plots and sub-plots consisted of bean lines and acaricides application, respectively. Population density of TSSM was recorded at weekly intervals, beginning from the seedling stage and continued to harvesting time on different lines. Yield loss caused by the mite on each line, was calculated by subtracting the average yields in sprayed subplots versus unsprayed subplots. For this purpose, 10 plants from each subplot were selected randomly and the yield and yield components including; the number of pods/plant, number of seeds/pod and weight of 100 seeds were recorded. Moreover, the grain yield (kg. ha^-1) was determined by harvesting the central area (10 m²) in each subplot. Our results indicated significant differences in TSSM population (eggs, juveniles and adults) among the studied lines. In 2009, the highest number of mite was recorded on line ‘L29' (120.20 ± 18.79) whereas, the lowest on line ‘L1' (22.51 ± 4.96 mites/2 cm² of the abaxial leaf surface). Moreover, during 2010, the highest and the lowest population density of the mite were supported by line ‘D3' (55.53 ± 5.68) and line ‘L1' (9.33 ± 0.79 mites/2 cm² of the abaxial leaf surface), respectively. Mean yield loss of pinto bean lines due to the mite attack calculated 1460.6 kg. ha^-1 (64.1 ± 10.2%) and 1278.7 kg. ha^-1 (61.4 ± 8.6%) in 2009 and 2010, respectively. The highest grain yield loss presented by line ‘L₁' (91.1 ± 12.1% and 86.4 ± 8.9%) followed by lines ‘L₂₉'and ‘L₃₀’, whereas, the lowest observed in lines ‘J₂₉' (41.5 ± 7.7% and 37.5 ± 7.6%) and ‘L₁₉’ (50.7 ± 6.6% and 47.4 ± 5.4% in 2009 and 2010, respectively).