Linear phototoxic furocoumarins were determined in fresh celeriacs using supercritical fluid extraction (SFE) and reversed phase liquid chromatography. The SFE method was optimized by comparing extraction yields at different temperatures and pressures, resulting in the density of the carbon dioxide between 0.3 and 0.9 g/mL. The addition of a drying agent (sodium sulphate) to the sample was important in avoiding the plugging of the linear restrictor used. The optimum yields of furocoumarins were obtained when the density of the carbon dioxide was 0.58 g/mL. The repeatability was enhanced by careful selection of the collection solvent; however, between the solvents tested, only small differences in their efficiency to retain the analytes were observed. The determination of furocoumarins from fresh material was more efficient, and difficulties were observed in the isolation of the analytes from freeze‐dried or air‐dried ground celeriac. The amounts of furocoumarins varied within the batches of tubers, the sum of three phototoxic furocoumarins being 2.1–4.5 mg/kg (on a fresh weight basis). However, the yields obtained by SFE were in agreement with those from Soxhlet extraction. © 1997 John Wiley & Sons, Ltd.