The occurrence of algae was investigated in surface water of the Arno river and in a drinking-water treatment utility serving the city of Florence. Throughout the 12 months monitoring period, raw water and treated water samples were collected and analysed for the number and species of microalgae and cyanobacteria; settled and filtered water samples were only examined for counts of organisms. Chlorophyll concentrations, temperature, pH, turbidity, flow rate and other indicators of surface water pollution were measured in raw water samples. Microalgae, mainly diatoms (46.1%, with a predominance of the genus Melosira) and green algae (35.1%: non flagellate 30.7%, green flagellate 4.4%) were constantly found in river water samples. Cyanobacteria were rarely detected except in summer, mainly of genera Oscillatoria and Anabaena. In treated water only depigmented and inactive algal cells, mainly of green algae, were detected for which the treatment procedure was found to be less effective. The average algal removal by drinking-water treatment resulted in 97.4% for all samples and in 98.3% when significant algal concentrations were present in raw water. The combination of chlorine dioxide and coagulants resulted in a removal of 90.1%.