This paper summarizes a poster paper presented at the International Symposium on Water Management held at Deakin University, December 2000. The study was carried out by 14 members of the Science Club of Ashimori Junior High School in Okayama City under the supervision of Principal T Segawa and Director R Noto. The Ashimori River runs through the north-west of Okayama City and has many suitable natural habitats in it for study. However, since 1994 a decline in firefly numbers has been linked to a decline in water quality. We counted aquatic fauna over several years and related it to physico-chemical characteristics of the water. We found some animals indicated better quality water and were principally found in the upper reaches of the river (e.g. Ephemeroptera, Trichoptera), although the number of Ephemeroptera declined over time. Pollution has increased steadily in the river over time.