We measured the anti-attachment activity of allelochemical and antifouling substances using monospores from Porphyra yezoensis Ueda as an assay. Methanol or aqueous extracts ($20/{mu}g/mL$) from 32 seaweeds were added to monospore suspensions. Methanol extracts of Corallina pilulifera, Ishige sinicola, Sargassum horneri, and Sargassum sagamianum inhibited attachment $by;>90\%$ compared to the reference. Phenolic compounds fractionated from S. sagamianum caused the most potent inhibition. P. yezoensis monospores also showed significant sensitivity to known allelochemical and algicidal compounds.