To better understand the antifouling action ofpara-substituted phenolic compounds, the antimicrobial activity, which is known to be related to the antifouling activity, was examined withTrychophyton mentagrophytes. The effect of phenolic compounds on mitochondrial swelling was also investigated to elucidate their membrane-perturbation action. The variation of each activity was quantitatively analyzed by a regression analysis, using hydrophobic substituent parameter π. Each activity varied parabolically with increasing hydrophobicity of thepara-substituent, the optimum π value beingca. 3.5–4.0. The introduction of a bulky substituent to theorthoposition resulted in a drastic decrease in each activity. It is therefore likely that the antimicrobial activity is due to perturbation of the membrane structure. Considering the good correlation between the antifouling and antimicrobial activities ofpara-substituted phenols, the antifouling activity is assumed to be related to perturbation of the membrane structure and/or function.