The objective of this study is to compare the engine performance and emission results of biodiesel derived from used cooking oil when applied in different proportions in outboard engines. Results revealed that the use of biodiesel resulted in lower emissions of CO (up to 12%) with an increase in emissions of NO, (up to 20%, except in one case which presented a slight reduction). Biodiesel also presented a slight increase in specific fuel consumption (lower than 11.4%) which may be acceptable considering the reduction in exhaust emissions. The experimental results proved that biodiesel alone or blended biodiesel can be used in compression ignition outboard engines, thereby providing a viable alternative to diesel. Special attention should be paid to the use of biodiesel in boats operating on lakes and rivers and in sheltered bays, which are more vulnerable to pollution. (c) 2006 Elsevier Ltd. All rights reserved.