Aerobic heterotrophic bacteria are isolated from the gut contents of midge larvae origin inhabiting the surface layer of the beds of slow sand filters (Hirabara works, Kure, Hiroshima). The isolates were divided into fermentative and non-fermentative groups using a differentiation medium (the OF basal medium, Eiken, Tokyo). Each group was identified by commercially available identification kits (Nissui, Tokyo); the EB-20 for the former and the NF-18 for the latter, respectively. The Enterobacteriaceae, Serratia and Vibrio groups were frequently identified from the gut flora as the fermentative members, while the Acinetobacter group was also found as the non-fermentative member. However, some of the isolates from a young larva eventually failed to be identified by the kits for clinically diagnostic purposes, probably due to the technical limitation based on the less data collection of naturally-occurring bacteria.