Legionella colonization in 31 cooling towers in or near Adelaide in South Australia was investigated. All towers were on identical cleaning and chemical treatment prorammes, including treatment with a monomeric quaternary ammonium biocide. Tower construction varied, wood, fibreglass and stainless steel models being included. Over a two-year period, water samples were taken from towers twice weekly and records kept of tower water pH, conductivity, total bacterial count, basin-water temperature, ambient temperature and hours usage of each tower at the time of collection. Temperature and tower usage showed direct correlation with Legionella counts. Between 60 and 75% of cooling tower waters were colonized by Legionella spp. during the summer months. This figure fell to 20-30% during the winter. Basin-water temperatures were shown to be a major determinant in Legionella colonization; regardless of ambient temperature, towers with elevated basin-water temperatures were frequently colonized. Analysis showed that Legionella counts in towers used continuously throughout the year were not influenced by ambient temperature.