Operation and maintenance costs have been compiled for 20 wastewater treatment plants in the range of 7 000 to 650 000 population equivalents. Great effort has been made to exclude the effects of local conditions on the costs. Data on energy and chemical consumption and manpower are given as well as the total operating costs and the distribution of manpower, energy, chemical and other costs. The results show that costs for manpower and electricity as well as total costs on a per capita basis decrease with increasing plant size. The variation between individual plants is, however, large even when differences in local conditions are accounted for.