Swimming is the most popular outdoor recreational activity. This conclusion has been reached by a Greater London Council survey of outdoor recreation (GLC, 1975) and by many other surveys. Yet even in the most enlightened cities, swimming is mainly available in special small pools, usually indoors. Coastal cities and river cities have polluted their natural facilities. This will not do. No one should have to leave their city to swim out of doors. Outdoor swimming is one of the Rights of Man. To make it available, almost everywhere, all we need do is adapt our procedures for water treatment and supply. "Safety first' proclaim the water engineers, forgetting that "covert enmity, under the smile of safety, wounds the world'. Water treatment is a staged process. If drawn from a river, water is allowed to settle in reservoirs, then filtered, then stored again, then distributed to customers. At one of these stages, water is perfectly suited to outdoor swimming. To make the swimming even better, the water body could be heated with waste heat from a power station. Impossible? Not if water engineers, power engineers, leisure managers and park designers were instructed to work together. This is far more practical than asking the wolf to dwell with the lamb, the leopard with the kid and the young lion with the fatling.