Bacteria forming biofilms occur in all open water cooling systems where they accelerate metallic corrosion, reduce flow rate and decrease heat energy transfer rate. A population structure study of seven systems was conducted. The isolate most frequently encountered was Pseudomonas fluorescens (35,5%), the species commonly used in research regarding biofilm formation. This was followed by Chromobacter violaceum, P. pickettii, P. stutzeri and P. putida, each amounting to 6,6%. The dominant organisms occurred in two groups of over 85% relatedness between their biochemical reaction patterns. Overall four distinguishable groups occurred on the 90% similarity level.