The development and preliminary use of two different molecular approaches for rapid enumeration and monitoring of ruminal prevotellas are described. Several oligonucleotide DNA probes, specific for the genus Prevotella and the species P. ruminicola and P. bryantii were labeled with various fluorochromes and used in in situ hybridization experiments. Epifluorescent microscopy was successfully used for the detection of fluorescent signal emitted by the probes in pure and mixed culture samples. The enumeration of the target cells and the analysis of the crude rumen fluid proved to be difficult, however, mainly due to the autofluorescent background and nonhomogeneous distribution of the cells on the microscope slide. The development of a competitive PCR system for ruminal prevotellas is described and the preliminary results of the rumen fluid analysis from a black-and-white Holstein cow are given.