DeLong, J. M., Hodges, D. M., Prange, R. K., Forney, C. F., Toivenon, P. M. A., Bishop, M. C., Elliot, M. L. and Jordan, M. A. 2011. The unique fatty acid and antioxidant composition of ostrich fern (Matteuccia struthiopteris) fiddleheads. Can. J. Plant Sci. 91: 919-930. The purpose of this study was to investigate the health-promoting composition of ostrich fern (Matteuccia struthiopteris) fiddlehead tissue by focussing on its fatty acid and antioxidant content and antioxidant activity. The curled crosiers (fiddleheads) were harvested following emergence and before 10 cm growth from eight or nine sites in eastern Canada during 2008 and 2009. The crosiers were then refrigerated or kept on ice until cleaned, subsequently frozen in liquid nitrogen, and then stored at -85°C. All tissue samples (except those used for ascorbate analysis) were freeze-dried, ground in a ball mill and stored at -80°C until analyzed. The current study showed that fiddlehead tissue had an unusual fatty acid composition including γ-linolenic, dihomo-γ-linolenic, arachidonic and eicosapentanoeic acids. The concentration of the antioxidant compounds ascorbic acid [3.0 µmol g^-1 dry weight (DW)], a- and γ-tocopherol (314 and 80.8 µg g^-1 DW, respectively) and a- and ß-carotene (43.8 and 122 µg g^-1 DW, respectively) and the xanthophyll pigments violaxanthin (225 µg g^-1 DW), zeaxanthin (127 µg g^-1 DW) and lutein (238 µg g^-1 DW), ranged from high to very high for green plant tissue. The phenolic compound content (51.6 mg gallic acid equiv. g^-1 DW) was also high compared with other fruits and vegetables and was likely responsible for the elevated antioxidant activity (1529 µmol trolox equiv. g^-1 DW; oxygen radical absorbing capacity assay) values recorded. Site differences were apparent for several of these measurements. Ostrich fern fiddlehead tissue appears to be a rich and unique source of antioxidant compounds, xanthophyll pigments and essential fatty acids.