In this study, trout oil methyl ester fuel (TOME) was prepared by transesterification using potassium hydroxide as catalyst. The trout oil and its blends (B10, B20, 840 and B50) were tested in a single-cylinder natural aspirated indirect injection diesel engine. The tests showed significant changes in engine's power and particularly torque as well as major improvements in the engine emission for B40 and B50 in general, except the increasing of nitrogen oxide (NOx) emission due to high combustion temperature resulted by better combustion process. The brake specific fuel consumption of B50 fuel was almost the same as that of diesel fuel at the maximum torque and rated power conditions. Carbon monoxide (CO) and hydrocarbon emissions (HC) were reduced around on average 13% and 45%, respectively, in case of TOME compared to diesel. (C) 2013 Elsevier Ltd. All rights reserved.