Five species of cyprinoid fish, Puntioplites proctozysron, Hampala macrolepidota, Labiobarbus siamensis, Barbodes gonionotus, and Henicorhynchus siamensis, were collected from Mae Ngad Somboonchon Reservoir, Chiang Mai Province from October 2002 to September 2003 and examined for parasites. Fourteen species of helminths were discovered: ten species of monogenea, Dactylogyrus macrolepidoti, D. protozysron, D. puntioplites, D. quadribrachiatus, Dactylogyrus sp I, II, III, Gyrodactylus sp, Paradiplozoon kamang, and P. krasoopensis; three species of metacercariae, Centrocestus caninus, Haplorchis taichui, and Haplorchoides sp and one species of nematode: Rhabdochona sp. The overall prevalence was 79.36% (373/470). The highest prevalence was found in Puntioplites proctozysron (90%, 81/90), and the lowest prevalence in Labiobarbus siamensis (60.64%, 57/94). Monogenea and metacercariae were found with the highest and lowest infection rates in the cool-dry and rainy seasons, respectively. Metacercariae of Centrocestus caninus and Haplorchis taichui are infectious to humans.