This paper describes a method for the determination of reduced sulfur compounds in hydrothermal seawater and body fluids from the hydrothermal tube worm Riftia pachyptila. Sulfur is a hey component of the hydrothermal ecosystem based on chemosynthesis, Sulfur compounds were derivatized at pH 8 (4.3 for sulfide in biological fluid) with a fluorescent reagent, monobromobimane, and separated by reverse-phase HPLC. The eluted compounds were detected by spectrofluorimetry. This method allowed: the quantitative analysis of sulfide, sulfite, thiosulfate, cysteine and glutathione in seawater, vascular blood and coelomic fluids from R, pachyptila, The detection limits were in the 0.1 mu mol l(-1) range with a precision lower than 10%, The method has been applied to hydrothermal seawater. The organisms are distributed along a gradient of sulfide (produced by the vent) and thiosulfate. Analysis of biological fluid was performed with a new sample treatment allowing the analysis of total sulfide (free and bound to haemoglobin) with results comparable to published methods.