Abstract: The present invention relates to a method for determining the relative abundance of individual species or lower phylogenetic subgroups of microorganisms in a mixed population of several microorganisms comprising the steps of: 1) providing a set of labeled in situ hybridization cluster oligonucleotide probes; 2) hybridization of said probes with a sample of the mixed population, and 3) quantitative analysis of the number of labeled microorganisms. Further it relates to a method for analyzing dynamics in relative abundance of individual microorganisms in a mixed population.