The development of environmentally nontoxic or non-polluting antifouling additives that can be formulated in practical coating requires assay involving target organisms. Described here are the simple laboratory assays that have been developed using the barnacle, Balanus amphitrite, a common fouling organism found throughout temperate and tropical seas. One of the assays depends on synchronous year-round mass culture, the procedure for which is described, of nauplii larvae and cyprids larvae. The laboratory assays provided quantitative estimates of toxicity and settlement inhibition of the test compounds. Laurinterol (1), isolaurinterol (2), alpysinal (3), and aplysin (4) have been isolated from the Korean red alga Laurencia okamurae. Their structures were identified by spectral data in comparison with the literature data. Compounds 1-4 inhibited larval settlement of the barnacle B. amphitrite with EC$_{50}$ values of 0.18- 36 $mu extrm{g}$/ml. Com-pounds 2-4 showed larval toxicity against nauplii of the barnacle B. amphitrite with 5-10 $mu extrm{g}$/ml, while laurinterol (1) exhibited no toxicity at even 100 $mu extrm{g}$/ml. Therefore, laurinterol was expected as a promising natural antifoulant.t.