Praziquantel has been used to control external parasites on marine fish but there is little data about its effect on treating external parasites infecting freshwater fish. This study examined whether praziquantel was able to control external parasites on freshwater fish in Vietnam. Four external parasites, monogenean trematode (Dactylogyrus sp.), parasitic Crustacea (Lenear sp.) and two ciliated Protozoa (Trichodina sp. and Epistylis sp.) naturally infecting Grass carp (Ctenopharyngodon idella), Common carp (Cyprinus carpio) and Tilapia (Oreochromis niloticus), respectively, were experimentally treated. Infected fish were exposed to praziquantel in baths for periods of 1, 3, 24, 48 and 72 hrs at each of five different concentrations 2.5, 5, 7.5 and 10 mg/l with a control (0 mg/l) treatment. Doses of 7.5-10 mg/l praziquantel administered for 1-3 h for fish infected with two ciliated Protozoa and 24-72h for the other two parasites were effective in eliminating all parasites without killing the fish. Thus, praziquantel of doses at 7.5-10 mg/l immersed within 72h duration is an appropriate therapy for simultaneous treatment of a number of external parasites of freshwater fish. The results have important implications in the management of fish health in current aquaculture systems.