In this study, the bacterial diversity in the activated sludge system of a full-scale municipal
wastewater treatment plant in Dubai was monitored over a period of one year using ribosomal RNA (rRNA)
targeted oligonucleotide probes for a defined phylogenetic group of bacteria by the Fluorescence in situ
hybridization (FISH) technique. The largest fraction of the bacterial community in the sludge samples belonged to the gamma-subgroup of proteobacteria (25%) followed by gram positive bacteria of high G+C content(16%), gram positive bacteria with low G+C content (9 %), beta-proteobacteria (8%) and alpha-proteobacteria(5 %) with respect to the population percentages stained by DAPI (4,6-diamino-2-phenylindole).A specific nocardioform actinomycete, simultaneously targeted by both HGC69a and MNP1 probes, was predominantly found throughout the study period in all activated sludge mixed liquor samples. The nocardioform actinomycetes group members were detected in both branched and single cell morphotypes. Most of the previously published genus and species specific probes failed to hybridize to the sludge samples. In conclusion, the overall bacterial community populations detected by the sub-group specific 16S rRNA targeted oligonucleotide probes in FISH technique remained almost constant throughout the period of study irrespective of treatment plant conditions.