Abstract A 13-kDa fatty acid binding protein (FABP) ( Fh13) has been isolated from the cytosol of adult Fasciola hepatica and its physicochemical and binding characteristics determined. Fh13 appears to exist as a dimer in native solution. Binding of the fluorescent fatty acid analogue 11-((5-dimethyl aminonaphthalene-1-sulfonyl) amino) undecanoic acid (DAUDA) to Fh13 results in changes in the emission spectrum, which are reversed by oleic acid. The binding activity for DAUDA determined from titration experiments revealed a single binding site per monomeric unit with K d of 1.5 μM. The displacement of DAUDA by competitive nonfluorescent ligands allowed K d values for oleic (2.5 μM), retinoic (2.8 μM), palmitic (4.1 μM) and arachidonic acid (6.1 μM) to be calculated. Ten commonly used anthelmintics were evaluated for binding to Fh13, but only bithionol showed binding activity commensurate with those of the putative natural ligands ( K d 6.8 μM).