Tissues from 23 Australian water rats (Hydromys chrysogaster) collected from five localities in central and northern Queensland, Australia, between February 1992 and May 1993, were examined for protozoan parasites and additional pathological changes. We found Klossiella hydromyos in the kidneys, Toxoplasma gondii in the brain and skeletal muscles and Sarcocystis sp. in the somatic musculature. Other pathological findings, including interstitial nephritis, interstitial pneumonia and a tongue abscess, as well as helminth-induced lesions in the lungs, mesenteries, stomach wall and cecal wall were also noted.