The bactericidal activities and factors affecting bactericidal activities were studied for five disinfectants, sodium hypochlorite, benzalkonium chloride, povidone iodine, saponated cresol and formaldehyde solutions, using killing-curve test on Aeromonas salmonicida, Vibrio anguillarum and V. ordalii. Three bacteria showed similar response to respective disinfectants. The dose to reduce viable cells of three bacteria from 10⁵ cells/ml to less than 10 cells/ml within a minute (decimal reduction time=15 sec)at 20°C were 10 ppm for sodium hypochlorite solution, 10-30 ppm for povidone iodine solution, 0.03-0.1% for benzalkonium chloride solution, 0.3% for saponated cresol solution and≥1.0% for formaldehyde solution. Cell number of bacteria and organic contamination in the solution strongly affected bactericidal activities of sodium hypochlorite and povidone iodine solutions. Bactericidal activities of benzalkonium chloride, saponated cresol and formaldehyde solutions were decreased remarkably by lowering the temperature of the solution. Water hardness reduced bactericidal activity of saponated cresol solution and seawater also reduced that of povidone iodine solution, when used as solvents.