Biodiesel is part of the family of biofuels and is a term used to define a fuel produced from a biological source (vegetable oils and/or animal oils/fats) that is used in lieu of diesel fuel. Biodiesel fuel is composed of fatty acid methyl esters (FAMEs) and is sold pure or as a blend with diesel fuel. Fire debris analysts are taught that most ignitable liquid residues they will encounter are petroleum-based liquids. This will likely be the case for many years to come. A few liquids, mostly oxygenated compounds and wood distillates (terpenes), are not petroleum-based and are encountered by fire-debris analysts sporadically. With the rapid emergence of alternative fuels, however, it is likely that unknown liquids or fire debris that contains such fuels will be submitted to the crime laboratory more often for examination. Biodiesel fuel is readily identified by gas chromatography-mass spectrometry. It can be extracted from fire debris samples by passive headspace concentration on activated charcoal strips; however, a solvent extraction that uses a nonpolar solvent is more suitable for this type of fluid. The analyst must be careful not to miss a possible mixture of biodiesel fuel with other ignitable liquid residues when interpreting the results of such analyses. 2 tables, 5 figures, and 39 references