Problem statement: Because of dramatic declines in stocks of brown trout in southern part of Caspian basin, the population's structure of brown trout (Salmo trutta) in several rivers were studied to provide data for conservation programs. Approach: The structure of the populations in the six rivers of the southern part of Caspian basin including: Keliyare, Khojirood, Lar, Shirinrood, Rig cheshme and Pajimiyane, were studied. Results: Five age classes, ranged from 0 + -4 + years, were determined. The most frequent age classes belong to 1 + and 2 + . The length ranged from 78-305 mm and weight ranged from 3.6-390 g. Also, the condition factor ranged from 0.58-1.47. The highest and lowest length, weight and condition factor were observed in Lar and Rig cheshme, respectively. In 5 out of 6 rivers, females were dominant over males. The highest and lowest female: Male ratios were observed in Pajimiane (6.75:1) and Khojirood (0.8:1), respectively. Significant relationships were found between total length of brown trout with depth (r = 0.6, p<0.0001) and width (r = 0.68, p<0.0001) of habitats in these studied areas. Conclusion: According to our knowledge, this is the first report of brown trout from Kelyare and Khojirood rivers. Since size of populations in studied areas are small and majority of these rivers located in low protected locations, it is essential to apply serious measures to protect these vulnerable habitats.