Seven species of fishes, 6 species of mollusks, 1 species of crab, and 1 species of shrimp were collected from 12 natural freshwater sites from 5 provinces: Lumpang, Phrae, Phayao, Chiang Rai and Chiang Mai during the Rainy season of 2000. Fishes, Channa striatus, Dermogenus pusillus, Oxyeleotris marmoratus, Trichogaster trichopterus and Trichopsis vittatus were examined for helminths and 34.69% (17/49) were infected. There were 2 genera of monogenean identified as follows: Dactylogyrus and Trianchoratus; 3 genera of metacercariae were as follows: Acanthostomum, Posthodiplostomum and Stellantchasmus. Furthermore, 1 genus of Acanthocephala was found from fishes to be Pallisentis sp. The prevalence of infection in mollusks was 6.20% (17/274); Pleurolophocercous and Furcocercous cercariae were observed only in one species of mollusks, Melanoides sp. A crab and a shrimp were negative.