Thiamin and riboflavin were determined in the sea urchin, Paracentrotus lividus, by reversed-phase liquid chromatography with fluorescence detection and a mobile phase consisting of 72% 0.005 M NH(4)OAc and 28% MeOH at a flow rate of 1.450 ml/min. The total analysis time never exceeded 20 min. After acid hydrolysis and enzymatic digestion, thiamin was oxidized to thiochrome and the oxidized fraction was purified and concentrated by solid-phase extraction. Limits of detection were 9 pg/ml and 80 pg/ml for thiamin and riboflavin, respectively. Good results were obtained with respect to repeatability (RSD%<2.4) and recovery (>91.3%).