Brown trout is a vulnerable flock of Caspian Salmonid fishes and usually spreads over most freshwaters upstream of Caspian catchments such as northern rivers of Iran. This fish has good supplies in Lar Reservoir of Mazandaran province, so that a number of 17862 entries of fishery permit were issued in 2006 (July to September). This study was carried out with the aim of taking knowledge of the morphological characteristics and its changes in Salmo trutta fario, to achieve this purpose 190 samples of red spotted trout caught from Elarm, Ab-Sefid, Kamardasht (Lar) and Delichaie of Lar Reservoir lake basin were studied using Electroshocker set in the fall of 2008 and summer and fall of 2009. The averages weight and fork length of studied fish were 136.86 ±103.4 g and 212.81±49.6 mm, respectively. The results show that these fish are descriptively (body color and spots, fins location etc) analogous to other red spotted trout fishes. A number of 17 variables were counted among which the number of scales on lateral line is 108-134, un-branched and branched rays in dorsal fin are 2-5 and 8-11, respectively and in anal fin are 2-5 and 6-9 respectively, the number of gill rakers are 15 to 21 and vertebrae are 57 to 60. From the assessment point view 31 factors were measured among them the averages of some factors are as follow: head length: 22.34±1.4, head depth: 14.80±1.1, maximum body height: 20.87±1.7, dorsal fin base length: 12.52±1.2, pectoral fin length: 15.60±1.2, ventral fin length: 12.39±1.0, anal fin base length: 9.38±0.9, pre-dorsal distance: 41.24±1.9 and pre-ventral distance: 46.84±2.3 percent of fork length. Meristic counts differences between males and females were not significant, but statistical differences between 18 morphological factors were significant. Differences were significant at a=0.001 in 9 factors, at a=0.01 in 5 factors and at a=0.05 in 4 factors. It is totally suggested that the results from morphometric and meristic data of the present study greatly differ from results from republic of Azarbayjan and seems to be a distinct population
