Abstract The effect of modification of aromatic and ionizable amino acid residues on the hemolytic activity of a thermostable direct hemolysin from Vibrio parahaemolyticus was examined. Tryptophan 65, one of the two tryptophan residues per subunit, was specifically modified with N-bromosuccinimide, resulting in complete loss of hemolytic activity. However, neither nitration with tetranitromethane of one of the nine tyrosine residues nor N Jm-ethoxyformylation of two of the four histidine residues caused any change in hemolytic activity. The hemolysin was fully active upon amidation of two reactive carboxyl group. On the other hand, acetylation of amino groups and the modification of one of the three arginine residues with 1,2-cyclohexanedione resulted in a partial loss of the hemolytic activity. The results suggest that Trp65 is essential for the hemolytic activity of V. parahaemolyticus hemolysin.