1. To understand the life cycle of an organism, it is important to understand the underlying physiological mechanisms of their life histories. We here use the theory of dynamic energy budgets (DEB) to investigate the close relationships between growth, reproduction and respiration in nematodes. 2. Using a set of simplified equations based on DEB theory, we are able to accurately describe life-cycle data from the literature for the free-living bacterivorous nematodes Caenorhabditis elegans, C. briggsae and Acrobeloides nanus, under different temperature or food regimes. 3. Nematodes apparently differ from other animals, as the initial growth is slower than expected. We explain this phenomenon by food limitation in the larvae, which is supported by more detailed physiological studies. 4. Food density and temperature are shown to have predictable effects on the growth curves (temperature affects only growth rate, whereas food density also affects ultimate size), although the reproduction patterns reveal some deviations from model predictions. 5. The presented model integrates the different aspects of the life cycle into a single framework, and can be applied as such to interpret the effects of various stressors. © 2005 British Ecological Society.