Epibiosis, the colonization of biogenic surfaces by epibiotic organisms such as bacteria, filamentous algae, and sessile invertebrates, poses a major threat to the fitness and survival of macroorganisms which could potentially be fouled. Fouling of artificial submerged structures can also cause severe economic problems, making the need for refined bioassays to determine the efficacy of potential antifouling compounds even more important. The aim of this study was to use the distinct swimming behaviour of zoospores of the fouling brown alga Hincksia irregularis to develop a new laboratory antifouling bioassay to test the effect of marine natural products. Spores were exposed to different concentrations of aqueous and organic extracts from body walls of sympatric echinoderms (Asteroidea: Luidia clathrata, Astropecten articulatus; Ophiuroidea: Astrocyclus caecilia). Computer-assisted motion analysis was used to distinguish between the straight and fast swimming movements of undisturbed spores (controls) and the helical and erratic swimming patterns of chemically irritated spores, using the quantitative parameters rate of direction change (RCD) and swimming speed (SPD). The ratio RCD/ SPD of spore swimming paths at extract treatments compared to controls can be used to quantify the detrimental effect of echinoderm extracts. Echinoderm extracts had significant effects on spore swimming behaviour at concentrations three orders of magnitude lower than that present naturally in the echinoderm body walls (mg extract/dry weight echinoderm body wall). Comparative studies on spore settlement and germination under similar treatment conditions show that changes in spore swimming behaviour reflect decreased fitness and survivourship of algal spores. It is suggested that this bioassay can be used to screen potential antifouling extracts and compounds at very low concentrations, making this assay particularly suitable for detection of concentration dependent effects and for bioassay-guided fractionation of extracts to identify active compounds.