Escherichia coli T even phages were ethanol sensitive, whereas T odd phages were ethanol tolerant. The phagecidal activities of alcohols on ethanol sensitive phages (T even, λ) were revealed to be in the order n-propanol > ethanol > methanol >> n-butanol >> n-hexanol >> noctanol. However, the those of alcohols on ethanol tolerant phages (T odd) were revealed to be in the order methanol > ethanol > n-propanol. The phagecidal activity of n-butanol (hydrophobic) increased when small amounts of hydrophilic methanol (10 and 20%, v/v), ethanol (10 and 20%, v/v) were added. The relative soluvbility curves of glucose plus Sudan III (hydrophobicity-hydrophilicity indication) were similar to the λ phagecidal activity curves of ethanol solution. The denaturation of ethanol-sensitive phage proteins with ethanol solution (50 or 70%, v/v) on SDS-polyacrylamide gel electrophoresis was associated with a loss of the plaque-forming activity of the phage. These phenomena were not observed for the ethanoltolerant phages treated with ethanol solutions. The head proteins of ethanol-tolerant phage have a greater number of hydrophobic amino acids and fewer hydrophilic amino acids, whereas those of ethanol sensitive phage have a greater number of hydrophilic amino acids and fewer hydrophobic amino acids. From these data, we suggest that the hydrophobicityhydrophilicity balance of the alcohol solution and the phage surface proteins affects the loss of the plaque-forming activity of E. coli phages by alcohol.