Information on natural concentrations or variability of secondary metabolites in marine organisms may be important both to ecological/evolutionary and applied approaches. A gas chromatographic procedure with an electron capture detector (GC-ECD) was developed to quantify the sesquiterpenoid elatol at the surface and within-thalli of 70 specimens of the red seaweed Laurencia obtusa. The concentration of elatol was highest within-thalli [9.89 mg g(-1) of L. obtusa, dry weight (d.w.)], compared to lower values found at the surface [0.006 mg g(-1) of L.obtusa (d.w.), or 0.5-10.0 ng cm(-2)]. This method provides a rapid and inexpensive quantification of small quantities of elatol, and probably may also be used to quantify other halogenated compounds usually found in red seaweeds.