ZR siru hybridization with fluorescently monolabelled rRNA-targeted oligonucleotide probes (17 to 18 nucleotides) was used to discriminate between Alcaligenes eutrophus JMP134 and Acinetobacter cnlcnucericus 69-V by flow cytometry. The strains were grown in batch experiments in a mixed popuhtion. The forward light scatter and fluorescence of each bacterial cell were measured with a single laser cytometer. The intensity of fluorescence after rRNA staining depended on the content of ribosomes, which cornelated with the growth rate of bacteria. Thetefore exponentially growing cells could be clearly detected. For other growth phases, signal amplification was necessary using multiple probes. The two bacterial strains were identified with differently labelled probes under an epifluorescent microscope. Using a single laser cytometer, rRNA based identi!ication was possible but not ideal. Better discrimination between the two strains of the mixed population was achieved by DNA staining, combined with the different forward light scatter signals. Due to the significantly different cellular DNA and GC content of both strains, the fluorescent dye DAPI (4'.6-diamidino-2-phenylindole), preferring AT-rich regions of DNA, was found to be ; I supplementary too1 for population analysis. The abundance ratios of the two strains in mixed culture determined by DNA or rRNA staining were similar.