1. We formulate a simple, physiologically based model to investigate the phenomenon of compensatory growth. 2. The model is based upon two key assumptions. First, that an individual partitions net assimilate between two tissue types: those which can and those which cannot be remobilized once laid down. Second, that the individual modulates its behaviour and physiology in response to the instantaneous ratio of mobilizable to non-mobilizable tissues. 3. The model is parameterized for salmonids from published studies of their energetics. Data from aquaculture studies of fish growth under conditions of fluctuating food are used to test the model. Using a common parameter set, the model successfully reproduces the growth patterns observed in 16 different feeding regimes.