Bacteria in the gut of midge larvae (Diptera, Chironomidae) living in the uppermost layer of the beds of slow sand filters (Hirabara works, Kure, Hiroshima) were isolated and characterized by their hydrolysis enzymatic activities against starch, casein and tributyrin, as compared with two other samples such as the surrounding sand beds and overlying water. Higher frequency (about 80%) of the isolates from the sand bed had hydrolytic activities against the substrates. This suggests that the midge larvae constantly produce faecal pellets which contain a number of bacteria having hydrolytic enzymes as a result of their grazing and excreting behaviours. Lower frequencies of those from the gut may be due to difficulty in culturing strictly anaerobes. The midge larvae and their faecal pellets are not accepted as unhygienic but beneficial factor, thus conditioning the sand beds to supply us with safe and more natural-tasty drinking water.