Abstract: A six month-study was conducted on five desalination units in Sharm El-sheikh region. The final desalinated drinking water, the high salt brine discharge water, water from Sharm El-Maiya Bay and other seven different types of natural drinking bottled-water were bacteriologically analyzed in order to; first investigate the degree of potability of the produced desalinated water, to assess the possible environmental impact of the brine discharge on the Bay water quality and living organisms and third to estimate the desalination efficiency of the studied units. Sharm El-Maiya Bay is a quite small water basin from which water is pumped to the desalination units and in which high salt brine is discharged. A total of 76 samples were tested for the presence of certain bacterial groups of hygiene and quality control importance. Total viable bacteria (TVB), total coliforms (TC), faecal coliforms (FC), faecal streptococci (FS), haemolytic faecal streptococci (HFS), salmonella and shigella (S&S), aeromonads (Aerom.) and vibrios counts cfu/ 100 ml were estimated using 0.45 µm bacterial filter technique on agar plates containing plate count agar and other differential and specific culture media. Salinity was measured in all samples. Bacterial count results showed unacceptable water quality for drinking. TVB counts ranged from 100-800 cfu/100ml, Aerom. 5-600 cfu/100ml, S and S 4-300 cfu/100ml, vibrios 1-27 cfu/100ml were very far from both the national and international standards. Bottled water reached acceptable standards. The results of bacterial counts in the discharged brine were 7-10 times more than those detected in the Bay water (control samples) which represent the feed water to the desalination units. Brine salinity measurements were at twice as high as these of the feed water. Both the high bacterial counts and salinity values may cause risky environmental changes in the Bay ecosystem especially on coral reefs, in fish aquoria, as well as to affect swimmers, snorkellers and scuba divers. Despite the high efficiency (100%) of the reverse osmosis membrane technology in removing seawater salts, the investigated desalination systems were not satisfactory in producing bacteriologically- safe potable water.