The paper describes a field experiment with the application of two biocides: oxytetracycline that reduced bacteria and captan that reduced soil fungi. The purpose of the experiment was to investigate the response of enchytraeid species to the changes of the abundance and activity of the microbial community as part of their food resources. The following variables were recorded: the biomass of microorganisms by the PLFA method, the rate of cellulose decomposition and the numbers and species composition of enchytraeids. No relationship was found between enchytraeids and abundance of microorganisms, but numbers of animals were correlated with changes in soil cellulolytic activity. Both biocides, even the non toxic oxytetracycline, reduced the diversity (H’) of enchytraeid community by increasing the proportion of dominant species. It is suggested that biocides reduced the functional diversity of microorganisms, and this factor had an effect on the enchytraeid populations. The animals responded to the treatment and their community became predominated by the species probably with a broad food spectrum.