The presence of algae in surface waters is a potential threat to the quality and ecology of the water system. The control and monitoring of the algae concentrations are therefore of high importance. Due to green algae problems in the water reservoir of a drinking water treatment plant, the ALGcontrol was installed to continuously monitor the algae concentrations and possible toxic blue green algae blooms. The results of the off-line standard lab method show a good correlation with the measurements of the ALGcontrol. According to these results the ALGcontrol gives in all cases reliable results, being a suitable, quicker and better instrument for continuous monitoring of algae in water.