DEMINERALIZATION using ion exchange resin is an important step required for production of ultra-pure water used in high pressure steam generation systems. This resin may get fouled in different ways resulting in loss of its efficiency. Such problem was investigated in Shoubra El-khiema and Damietta power plants in Egypt. Physicochemical analysis of the resin showed decrease in total exchange capacity of the used resin in the two plants. Chemical and microscopical examination revealed fouling by organic matter and iron. Resin scanning using electron microscope showed growth of bacilli bacteria adsorbed on its surface. Epifluorescence stains showed the presence of living microflora on the new resin, as well as living and dead ones on the used resin. Culture on different media revealed the growth of bacteria on nutrient and blood agar, but not on MacConkey's Agar. Fungal growth was observed on Sabouraud dextrose agar and Czapek's dox agar. Eight gram positive bacterial isolates were isolated from the used resin of the two plants, all of them were bacilli. Sequence analysis followed by phylogenetic investigation showed that seven isolates belonged to Bacillus sp., whereas one isolate was identified as Brevibacterium frigoritolerans CMG M5. The fungal isolates differed in the two plants, and included Penicillium sp, Fusarium sp, Aspergillus flavus, Aspergillus niger and Alternaria sp.