Abstract: In present study, the effects of sublethal doses (10 and 20 mg L-1) of Roundup® on fatty acid pattern in muscle and liver of brown trout were investigated. For this purpose, fish were held in experiment tanks for 1 month. While total MUFA wasn’t influenced, the highest total SFA and total n-6 PUFA were determined in group 10 mg L-1 and the lowest values were determined in control group and group 20 mg L-1 in muscle, respectively. The highest and the lowest total n-3 PUFA was found in control group and group of 10 mg L-1 in muscle, respectively. Total n-3/n-6 PUFA ratio and EPA+DHA level of group 10 mg L-1 were lower than other groups in muscle. The amount of total n-3/n-6 PUFA, EPA+DHA and total n-3 PUFA of control group were found higher than treatment groups in liver. While the highest total SFA was determined in group 10 mg L-1, there wasn’t difference between control group and group 20 mg L-1 in liver. Both of doses herbicide had higher value than control for total MUFA in liver. While Roundup® didn’t inhibit n-3 PUFA synthesis in the muscle, both concentrations, exhibited inhibitory effect on n-3 PUFA synthesis in the liver. This result probably consequence of its indirect effect on the some enzyme activities or gene expressions in fatty acid metabolism of brown trout.