An investigation of helminths in the Norway (brown) rat, Rattus norvegicus, and roof rat, Rattus rattus, from Chiang Mai Moat during May to August 1995, was done. Thirty-three out of thirty-eight trapped rats were infected (86.84%); 16 R. norvegicus (100%) and 17/22 R. rattus (77.27%). The rat was infected with 10 helminth species: 4 trematodes, Centrocestus sp (2.63%), Echinostoma ilocanum (10.52%), Echinostoma malayanum (10.52%) and Quinqueseralis quinqueseralis (39.47%); 2 cestodes, Raillietina sp (36.64%) and Taenia sp (cysticercus) (7.89%); and 4 nematodes, Angiostrongylus cantonensis (42.10%). Nippostrongylus sp (34.21%), Rictularia sp (52.63%) and egg of Capillaria hepatica (7.89%). The helminths were found in the small intestine (84.21%), large intestine (42.10%), lung (36.64%), stomach (28.94%), heart (23.94%), and liver (15.78%). The female Norway rats were infected with 10 species of helminths and the males with 6 species. In the roof rat, 7 species of helminths were found in females and 6 species in males.