Suspensions of legionellae and associated bacteria found in a hot water distribution system were maintained in the exponential phase of growth without addition of nutrients by constant agitation and oxygenation and by periodically transferring them to autoclaved tap water. The suspensions were exposed to different temperatures to determine growth and inactivation kinetics of legionellae kept under conditions similar to their natural environment. Multiplication of the legionellae was found to occur in a temperature range between 20 and 43 degrees C and inactivation was observed above 50 degrees C. Decimal reduction times decreased with increasing temperatures. These findings do not support the hypothesis that naturally occurring legionellae are more heat resistant than strains of the bacterium cultured on artificial media. Frequent failure to eradicate legionellae from hot water systems by elevating the water temperature indicates that it is impossible to achieve effective temperature levels concomitantly in all parts of the system.