The aim of this paper is to illuminate the ecological interplay between the city and countryside by studying the flow of phosphorus from the ecosystem to the community (settlement area) and back to the ecosystem in the Swedish municipality of Gävle. Approximately one third of the phosphorus passes through the system while two-thirds are accumulated, mainly at waste dumps—municipal and industrial. The inflow to the area is approximately 330 tons annually, or 3·7 kg per person per year. Of the 1·3 kg of phosphorus per person per year leaving the system, approximately half goes into the Baltic and the rest is exported in the form of agricultural products, fish, paper or oil. The human nutritional needs of phosphorus are of the same order as the flow of phosphorus going into the Baltic. The paper deals with ecological, economic and technical solutions to increase the amount of nutrients returned from cities to forests and agricultural systems. Possible development of our present sewage system, similar to that which has taken place in the solid waste system, is also described.