Rainbow trout ( Oncorhynchus mykiss ) is an exotic fish in the inland waters of Iran and little is known about this species in Iranian waters. In this study, some biological characteristics of rainbow trout were examined in 355 specimens collected monthly from the Beheshtabad River of Chaharmahal-va-Bakhtiari Province during April 2013 to March 2014. Total length of females and males ranged from 17.1 to 40.2 cm (26.05±4.54 SD) and from 15.5 to 40.3 cm (24.9±5.93), respectively. Among all the examined stomachs, 39 were empty (Vacuity index, Vi= 10.98%). This variation was not significantly diffrent (P?0.05) over the year and the maximum number of empty stomachs was recorded during the spring and the minimum during the autumn. The diet of the rainbow trout was composed of 45 different prey taxa, belonging to 10 main 0.0094L 3.063 , (r 2 =0.96) for males and W= 0.054L 3.087 , (r 2 =0.944) for females. This relationship showed that the growth was isometric for both sexes. The von Bertalanffy growth models were estimated as Lt=52.04[1-e -0.1582(t - 1.85) ] and Lt=66.39[1-e -0.105(t-2.46) ] for males and females, respectively. The growth performance indices were estimated as 6.06 and 6.14 for males and females, respectively, indicating a faster growth rate for female individuals. Keywords: Fecundity, Feeding habits, Growth, Oncorhynchus mykiss , Reproduction, Salmonidae Rainbow trout ( Oncorhynchus mykiss ) is an exotic fish in the inland waters of Iran and little is known about this species in Iranian waters. In this study, some biological characteristics of rainbow trout were examined in 355 specimens collected monthly from the Beheshtabad River of Chaharmahal-va-Bakhtiari Province during April 2013 to March 2014. Total length of females and males ranged from 17.1 to 40.2 cm (26.05±4.54 SD) and from 15.5 to 40.3 cm (24.9±5.93), respectively. Among all the examined stomachs, 39 were empty (Vacuity index, Vi= 10.98%). This variation was not significantly diffrent (P?0.05) over the year and the maximum number of empty stomachs was recorded during the spring and the minimum during the autumn. The diet of the rainbow trout was composed of 45 different prey taxa, belonging to 10 main 0.0094L 3.063 , (r 2 =0.96) for males and W= 0.054L 3.087 , (r 2 =0.944) for females. This relationship showed that the growth was isometric for both sexes. The von Bertalanffy growth models were estimated as Lt=52.04[1-e -0.1582(t - 1.85) ] and Lt=66.39[1-e -0.105(t-2.46) ] for males and females, respectively. The growth performance indices were estimated as 6.06 and 6.14 for males and females, respectively, indicating a faster growth rate for female individuals. Keywords: Fecundity, Feeding habits, Growth, Oncorhynchus mykiss , Reproduction, Salmonidae