Two major problems associated with biofouling studies are the lack of broad scale testing and failure to use consistent standards among different assays or studies. To address these issues the activity of two biologically active natural products, elatol and deschloroelatol, isolated from the marine red algaLaurencia rigida, and three commonly used biocides, Nopcocide N‐96™, Irgarol 1051™ and Sea‐Nine 211™, was compared, in a broad spectrum of bioassays. The activity of the different compounds varied substantially among different bioassay tests. Elatol and deschloroelatol had a narrow range of activity with strongest effects against invertebrate larvae. Both compounds were highly toxic. However, neither compound had strong activity against marine bacteria or the common epiphyteUlva lactuca. Irgarol 1051 also had a narrow range of activity, only affecting algal settlement strongly. Nopcocide N‐96 and Sea‐Nine 211 had moderate to strong activity across the spectrum of bioassays,viz.growth of marine bacteria(Vibrio fischeri, Serratiasp.), inhibition of settlement of macroalgae(Ulva lactuca), toxicity(Balanus amphitrite), and inhibition of settlement of invertebrate larvae(Balanus amphitrite, Bugula neritina). Based on the results it is proposed that Sea‐Nine 211, because of its broad spectrum activity, be used as a standard for comparative assessments of the antifouling activity of marine natural products and analogues.