Residual fish oil, which is basically a by-product of fish processing industry, was tested pure and in 50% v/v blend with conventional low-sulfur marine gas oil in a direct-injection heavy-duty diesel engine. Experiments were performed at various operating conditions under standard propulsion and generator mode marine cycles. Engine performance, exhaust smokiness, gaseous emissions together with particulate matter (PM) size distribution and corresponding total particle number and mass concentrations were measured in all of the tests performed. In general, fish oil showed fairly good combustion and ignition properties, which were very similar to that of marine gas oil. Only slight difference was observed in NOx and CO2 levels, while emissions of CO and THC were significantly decreased at all operating conditions. Fish oil also allows reducing smoke (soot) and both total number and PM mass emissions up to 70-90% with the biggest particles being most affected. Such positive impact on pollutants can be explained by high oxygen content in fish oil fuel and its lack of aromatic compounds, which is promoting more complete fuel oxidation. (C) 2012 Elsevier Ltd. All rights reserved.