This paper describes a model of the metabolic cost of swimming in pinnipeds and its application to other marine homeotherms. The model takes account of both hydrodynamic and thermal processes. The thermal component incorporates both free and forced convection and takes account of the effect of hair on free convection. Using data from the literature to evaluate all but two of the parameters, we apply the model to metabolic rate data on phocid seals, otariids (sea lions), penguins and minke whales. We show that the model is able to reproduce two unusual features of the data; namely, a very rapid increase in metabolic rate at low velocities and an overall rise in metabolic rate with velocity which is slower than the rise in hydrodynamic drag force. The work shows the metabolic costs of propulsion and thermoregulation in a swimming homeotherm to be interlinked and suggests differing costs of propulsion for different modes of swimming. This is potentially of ecological significance since the swimming speed that minimises the cost of transport for an animal will change with changes in water temperature.