The effect of larval bibionid feeding on microbial respiration (CO2 production and O-2 consumption), fluorescein diacetate activity (FDA), direct counts of bacteria and composition of the microbial community were studied. Leaf litter and larval excrement (1-3 days old) were used as food. Two species of bibionid larvae and two types of leaf litter (Bibio marci feeding on Carpinus betulus litter and Bibio pomonae on Populus nigra) were used. Gut passage resulted in an increase of microbial O-2 consumption, total microbial metabolic activity, direct counts of bacteria and number of actinomycetes in the fresh excrement. This effect was only short term. Reduction of bacterial counts and shift in composition of microbial communities were observed during both the first and second gut passages. Passaging induced decrease in the C and N contents of the substrate, the C/N ratio and the amounts of water soluble carbohydrates. Animal - microbial interactions differed between the first and the second gut passage. During the second gut passage higher numbers of bacteria in the gut content were found. This appears to be due to the higher counts of bacteria in the food and a higher proportion of bacteria which could survive gut passage. The effect of gut passage was dependent on the species of bibionid larvae and on the food that was consumed. CO2 production responded differently than O-2 consumption and most of the microbial parameters suggesting that CO2 production is influenced not only by microbial activities but also by other factors (aeration status, pH).