The efficacy and the potential toxicological impact of a proposed ballast water treatment (PERACLEAN Ocean) using peracetic acid (PAA) as active substances to control species introduction was assessed in both fresh- and salt water experiments at very cold water temperatures (1-2 degrees C). Levels of PAA gradually declined over the 5-day experiments, while levels of hydrogen peroxide remained relatively stable. The rate of decay of both the PAA and hydrogen peroxide in water was accelerated in the presence of sediments. Water quality properties varied significantly with treatment level with a maximum reduction of pH by 2.0 units and a concomitant 20-fold increase in dissolved organic carbon levels. Living biomass of organisms in treated water was reduced by 99% after 2 days. Results from six toxicological tests revealed very steep dose-response curves of the treatment. The toxic response of treated waters was higher in fresh water than in salt water. The PERACLEAN Ocean treatment may represent an effective technology to treat ballast waters under a wide range of temperature and salinity conditions. The discharge of treated fresh water may however pose some toxicological risk to fresh water receiving environments and to cold waters in particular.