Raw water supplies utilised at 12 fossil-fired power stations, as well as the corresponding open recirculating cooling water systems were surveyed. Visual inspections were carried out and the total aerobic and anaerobic bacteria, anaerobic acid-producing bacteria, Thiobacillus spp., Nitrobacter spp., sulphate-reducing bacteria (SRB) and algae were quantified. All raw water supplies and recirculating cooling waters contained all of the above groups of micro-organisms, with the exception of the two potable raw water supplies. In 75% of the systems, the numbers of SRB in the recirculating cooling waters were higher than in the corresponding raw water supplies and in 92% of the systems, the numbers of the total aerobic bacteria were higher in the recirculating cooling waters than in the raw supplies. However, no relationship between the sulphate levels in the recirculating cooling waters and the numbers of SRB could be distinguished, or between the percentage increase in the numbers of total aerobic bacteria and the cycles of concentration at which the system was operated. The frequency polygons of the occurences of total aerobic and anaerobic bacteria in the raw water supplies and the recirculating cooling waters did not follow normal distribution patterns. Visible biofouling deposits were observed at six of the power stations surveyed and the predominant algal group was the blue-green algae. However, in the raw water supplies, the predominant algal groups were green algae and diatoms. Microbiologically influenced corrosion was identified in all five of the condensers inspected. Each system was found to be unique and no generalisations in terms of presence or activity of micro-organisms could be made.