The use of comparative ribosomal RNA sequencing for studies in microbial ecology and microbial evolution is discussed. A specific study, using the 16S ribosomal RNA to monitor the bovine rumen microbial community, is described. The response of this community to antibiotic addition was evaluated by hybridization of the oligonucleotide probes to nucleic acid isolated from the rumen. The application of the ribosomal RNA based measure of microbial communities to general evaluations of community disruption, as might follow the release of genetically engineered microorganisms into the environment, is discussed. (Copyright (c) 1988 American Chemical Society.)