Motivated by problems where variation among individuals is necessary to explain properties of ecological systems, the authors develop a mathematical model of an individual organism. The model, based primarily upon energetics, is developed specifically for female daphnids, although with appropriate modifications it should be applicable to other aquatic animals such as fish. Mimicking the life history of an individual as it progresses from egg to juvenile to adult instars, the model consists of a coupled pair of nonlinear, nonautonomous ordinary differential equations. The growth of an individual is described through the dynamics of two compartments--lipid and structure--of the organism because of the importance of lipid dynamics in aquatic animals. Energy supply and demand are handled through an energy integrator compartment. Availability of energy is represented by potential flows from the lipid and structural compartments. Energy requirements of maintenance, activity, and reproduction are the sinks. An advantage of employing an individual-based technique is that ample information relating to the individual parameters--most of which are physiological in character--can usually be obtained, and reasonable estimates for model application found. (Copyright (c) 1990 by the Ecological Society of America.)