ABSTRACT: To understand the role of marine natural products in surface mediated ecological interactions--such as biofouling--we need to be able to quantify metabolites at or near the relevant surfaces. We describe a new technique for determining the concentrationof natural products on the surface of marine algae. Surface metabolites were quantified for 2 red algae, Delisea pulchra and Laurencia obtusa, by dipping the algae in hexane for 20 to 40 s at room temperature. This allowed for quantitativeextraction of non-polar natural products from the surface of the thalli without disrupting cells (as determined by epifluorescence microscopy). More stringent extraction procedures using other solvents, or longer extraction times in hexane (>50 s), causedsignificant quantifiable cell damage. Natural products in the surface extracts were then measured using gas chromatography-mass spectroscopy (gc-ms). Mean total surface concentration of natural products from D. pulchra were 250 ng cm^-2,but <1 ng cm^-2 for L. obtusa. These results contrast to whole plant levels of total secondary metabolites in the 2 algae, which were significantly higher in L. obtusa (7 µg mg^-2 dry weight) than in D. pulchra(3.4 µg mg^-2 dry weight). Dipping thalli in hexane for 30 s also caused no cell lysis in 8 other species of macroalgae. This suggests that the procedure is more broadly applicable for the quantification of non-polar surface metabolites onseaweeds, and other organisms with resistant surface cells. Our results highlight the need to determine where compounds occur, and at what concentration, if we are to understand their ecological roles.